Remove final message
Make sure cleanup always happens

Setting the `trap` should be the first thing done immediately after acquiring
the lock.
Replace backup-status with a -s/--status command

It's cleaner to keep all the stuff in one script, anyway.

This new version allows the user to specify a message to print if a backup is
happening. It exits 0 if a backup is in progress, 1 otherwise. The given message
is only printed if a backup is happening.