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.
The fewer unnecessary changes the better.  I'd go with:
```ruby
raise Puppet::Error, "name must be an absolute path" if value =~ %r{^[^/]} or value =~ %r{/\.\./}
```