pg_hba.conf host name wildcard support

So, as previously indicated, let's add some wildcard support to the
pg_hba.conf host name feature.  After looking around a bit, two syntaxes
appear to be on offer:

1. TCP Wrappers style, leading dot indicates suffix match.
So .example.com matches anything.example.com.  Not sure how useful that
would be, but it could be implemented in about 3 lines of code.

2. Full regular expressions.  I'd suggest the pg_ident.conf style, where
a leading slash indicates a regex.  An example could be /^dbserver\d\.
With some code refactoring, this would also only take a few extra lines
of code.

Comments, other ideas?