Various IAT related changes.

 * 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.