usermap regexp support

The attached patch tries to implement regexp support in the usermaps
(pg_ident.conf).

The use-case will be to do things like realm-based matches in
kerberos/GSSAPI authentication (will require some further hacks on that
one to expose the realm name). For example you could have:

krb /^(.*)@myrealm.com$ \1
krb /^(.*)@otherrealm.com guest

and things like that.

It will also likely be quite useful once we get SSL certificate based
authentication.

Reviews particularly appreciated - I really can't claim to *know* our
regexp code :-O

Obviously docs need to be updated as well.

//Magnus