Improve method of determining the last day of the month.

This is a bit more resilient for two reasons:
1. It doesn't depend on cal which some systems don't have by default.
2. More importantly, cal on some systems highlights the current day of
   the month. This MUST be disabled with the "-h" parameter or escape
   sequences will be included in the output. This will break the check
   as to whether this should be a monthly backup (it'll be a daily) and
   arguably worse will break the deletion mechanism resulting in some
   backups never being automatically removed and costing the user money.
Merge branch 'fix_ldom'

* fix_ldom:
  Improve method of determining the last day of the month.