Support for hooking an external script

[vstefanoxx]

Hi.

Considered that is difficult to add support for ACL synchronization to bsync, I propose to add support for running an external script between apply_small_actions and apply_rsync_actions.
The external script could take care of ACL synchronization, and if I'm not wrong the best moment to do it is between the fast moves and deletion (apply_small_actions) and the possible slow rsync operations, to reduce the time window where ACL are not synchronized.
If using --acls rsync argument, is not needed to resync ACL after rsync operations.

The external script could be specified by a command line option -s, for example.

If you agree, I can branch and propose a pull request.
(I did now another pull request for batch mode and autoanswer, as discussed in another thread)

Stefano