Minor tweak to hostname check for Cygwin environment support

I'm pleased to say your script works perfectly on Cygwin except for the
hostname check. Under Cygwin the hostname command has no parameters and
so the "-s" option fails. However, it returns a short name by default.

Some Cygwin environments may also run into a problem due to the "cal"
program not being included, however, this can easily be fixed via
installing the util-linux package.
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.