Better regular expressions for mountpoint paths.

This changeset modifies the `mountpoint` regular expressions which determine absolute paths, to not error out in a couple of cases that I could find. I was trying to use two-character directory names - to wit, `/var/lib/pg/data` - and this regex was choking on them.

I've tried to make a general solution that works for all the forms of expressing a path.
These should have been left without `== 0`. The first regular expression already restricts itself to the beginning of the string with `^` and the second regular expression should be allowed to match anywhere in the string.  You don't want to let `/mnt/location/../otherlocation` through, for example.