Improve System Timer Usage

[mole99]

Magic has (ideally) two timers running, the backup timer creates a backup every ~5 minutes or after a certain number of changes, and some long running functions set up a timer for progress checks. When a new timer is set up, it interferes with the one that is already setup (at least on Linux). This can cause the progess checks to stop, or worse, the crash backup to stop.

Possible solution: use create_timer() instead of setitimer()