[rdiff-backup-users] lock file for backup directory

[Sebastien Maret]

Listing the increments in a backup directory while an other backup is in
progress give the following error:

rdiff-backup -l host::/path/to/backup
Fatal Error: Previous backup to /path/to/backup seems to have failed.
Rerun rdiff-backup with --check-destination-dir option to revert directory to 
state before unsuccessful session.

The message is confusing, since the previous backup hasn't failed,
but it just hasn't finished yet.

Beside, I am not sure if doing other operation (like delete old
increments) while a backup is in progress is safe. If not, it might be
useful to implement a lock system, to prevent two different
rdiff-backup processes to modify a backup directory simultaneously.

Sébastien