Added darcs apply as a restricted command

Darcs [http://darcs.net] can use scp and sftp to fetch patches from
a remote repository, however in order to commit patches, Darcs
needs to execute the darcs binary using ssh.

This patch adds darcs as a restricted command to rssh.
Restricted to execute only valid apply commands as sended by
the darcs program.

We are using this patch at darcs.thehatcher.com to restrict
access to darcs commiters.

--
vborja(at)thehatcher.com