ClientFactories now have a Dial() method instead of a WrapConn()
method, so that it is possible to write something like meek-client
using the obfs4proxy framework.

This breaks the external interface if anyone is using obfs4proxy as
a library, but the new way of doing things is a trivial modification,
to a single routine that shouldn't have been very large to begin with.

Differences from my goptlib branch:
 * Instead of exposing a net.Listener, just expose a Handshake() routine
   that takes an existing net.Conn. (#14135 is irrelevant to this socks
   server.
 * There's an extra routine for sending back sensible errors on Dial
   failure instead of "General failure".
 * The code is slightly cleaner (IMO).

Gotchas:
 * If the goptlib pt.Args datatype or external interface changes,
   args.go will need to be updated.

Tested with obfs3 and obfs4, including IPv6.

Unless you have very good reason to do so, there should be no reason to
actually call these ever, since the log messages are only generated if
they will result in output being written to a log file.

Implements feature #15576.

If the relevant enviornment variable is set, treat read errors from
Stdin as a SIGTERM.

This combines the old signal processing code with the parent monitor,
into a new termination monitor structure, which also now handles keeping
track of outstanding sessions.

The ideal solution here would be to implement #15435, but till then
use one of several kludges:
 * Linux - prctl() so that the kernel SIGTERMs on parent exit.
 * Other U*ix - Poll the parent process id once a second, and SIGTERM
   ourself/exit if it changes.  Former is better since all the normal
   cleanup if any gets done.
 * Windows - Log a warning.

The Go developers decided to move the go.net repository to
golang.org/x/net, and also to transition from hg to git.  This wasn't
changed when the go.crypto imports were since the 'proxy' component
doesn't have imports that break, so the old code still works.

While the change here is simple (just update the import location), this
affects packagers as it now expects the updated package.  Sorry for the
inconveneince, I blame the Go people, and myself for not just doing
this along with the go.crypto changes.

This allows obfs4proxy to be used as a ScrambleSuit client that is wire
compatible with the obfs4proxy implementation, including session ticket
support, and length obfuscation.

The current implementation has the following limitations:
 * IAT obfuscation is not supported (and is disabled in all other
   ScrambleSuit implementations by default).
 * The length distribution and probabilites are different from those
   generated by obfsproxy and obfsclient due to a different DRBG.
 * Server support is missing and is unlikely to be implemented.

Forgot to include this in the spec, though it was documented as a
comment in the framing code.

The Go developers decided to move the go.crypto repository to
golang.org/x/crypto, and also to transition from hg to git.  The tip of
tree code.google.com copy of the code is broken due to the import paths
pointing at the new repository.

While the change here is simple (just update the import location), this
affects packagers as it now expects the updated package.  Sorry for the
inconveneince, I blame the Go people.

Exhaustively testing padding combinations is really slow, and was
causing timeouts during the Debian ARM package build process.  Attempt
to improve the situation by:

 * Reusing the client and server keypair for all of the tests, to cut
   runtime down by  ~50%.
 * Splitting the client side and server side tests up, as it appears
   the timeout is per-test case.

If this doesn't fix things, the next thing to try would be to reduce
the actual number of padding lengths tested, but that is a last resort
at the moment.

Instead of "node-id" and "public-key" that are Base16 encoded, use
"cert" which contains the "node-id" and "public-key" in Base64 encoded
form.  This is more compact and cuts the length down by 49 characters.

Write an example client bridge line suitable for use with the running
obfs4 server instance to "obfs4_bridgeline.txt" for the convenience of
bridge operators.

Client side logs are less spammy than server side in general, so more
messages should be visible at the default logLevel when running as a
client.

Server side logging will be spammy basically no matter what unless
obfs4proxy gets into the (arguably dangerous) business of figuring out
which errors are people being evil vs which ones are transient network
issues, so most logging is suppressed by default, unless the admin
choses to open the floodgates.

The prolog prints the version as soon as logging is enabled, but before
the pluggable transport configuration is done.  The epilog is printed as
the code returns from main, as long as either client or server pt
configuration succeded.

For consistency with the rest of the arguments.

By default logging will be done at the "WARN" level.  Fatal
initialization errors will always be logged as long as logging
is enabled regardless of logLevel.

Instead of omitting errors entirely when running with the log scrubber,
filter common network errors through elideError() that can scrub the
common net.Error types and remove sensitive information.

 * Unbreak inbound TYPE_PRNG_SEED processing.

 * IAT obfuscation is now a per-bridge argument (iat-mode).
   * 0 (default) = Disabled.
   * 1 = Enabled, ScrambleSuit-style with bulk throughput optimizations.
   * 2 = Paranoid, Each IAT write will send a length sampled from the
     length distribution. (EXPENSIVE).

The "iat-mode" argument is mandatory on the Bridge lines, and as a
ServerTransportOption.  Old statefiles will continue to load and use
the default value, edit it if your hat is made of tin foil.