PAM: allow authenticated_as=user_name mapping

extend pam_user_map.so to support syntax like

1

*:*

2

@group:*

the first line will set authenticated_as to be the same as user_name, the second line will do that only for users in the specified group. That is, one can read * as the "authenticated pam user name".

The goal is to allow

CREATE USER ''@'%' IDENTIFIED VIA pam

but have users logging in with CURRENT_USER=USER(), not CURRENT_USER=''@'%', in other words, to support user management outside of MariaDB