Add recording of logfile to kup-daemon

Log is kept in users cache folder, only stores which commands get executed and stderr output from the commands. File is cleared at start of backup saving so it only ever shows the last run of a backup plan. One file for each backup plan.
Modify notification to include a "show log file" button. Add menu item for showing log file.
Closes #1.
Make libgit2 close packfile file descriptors when done.

A problem is that bup archives can easily have over 1000 pack files, which is not normal for a git repo. libgit2 has not been designed for that, it keeps file descriptors open (presumably as an optimization).
This is a bit hackish fix since it's only done in one place. Seems to be enough for now.