daemon: remove from db files with dtime older than a given date.

daemon: remove from db files with dtime older than a given date.

LightMediaScanner will not delete the database entries for files that
are gone, instead it will mark the 'dtime' column with current
timestamp. This allows fast restore of files from a removable media,
but can grow the database too much if these are never purged from time
to time.

This commit introduces --delete-older-than=DAYS (-d=DAYS) that will
always purge entries older than the given number of days, or negative
value to disable this behavior.