lib/vserver: added vservers_running function, use it in sys handler (to ease future fix for Trac#45)

dup: support every duplicity-supported transport with new configuration option desturl (Closes: #483712, #346040, Trac#2)

ssh identity file, defaulting to the expected /root/.ssh/id_rsa. This
is important to have if you are trying to backup to a different user
on a backuphost than other handlers are using. For example, if the
rdiff handler is connecting to backalbatross@backupserver-pn using
/root/.ssh/id_rsa for public key authentication, and you want to
configure the maildir handler to connect as user
backmaildir@backupserver-pn you will might want to specify a different
identity file for that authentication (if you do not want to share the
id file between the 'backalbatross' and 'backmaildir' users). 

Note: this cannot be accomplished with a ssh config file because that
file is host based, ie. you can only specify one user per host
specified in the config

fixed bug where maildirs that start with a number were skipped. make deleted maildirs record the date they were deleted

find: warning: you have specified the -maxdepth option after a
non-option argument -type, but options are not positional (-maxdepth
affects tests specified before it as well as those specified after
it).  Please specify options before other arguments.

 - general cleanup of dup handler
 - support remove-older-than command
 - also run "duplicity cleanup" before anything else, for safety reasons
 - migrated full/incremental backup switch to new syntax
ChangeLog: removed trailing space