add group support to pam_user_map module

Currently pam_user_map module (plugin/auth_pam/mapper/pam_user_map.c) can map user names based on the simple configuration file, like

#comments and emty lines are ignored

john: jack

bob:  admin

top:  accounting

Here the user "john" will be renamed to "jack".
This task is to add support for mapping, based on the user group, like:

#comments and emty lines are ignored

john: jack

bob:  admin

top:  accounting

@group_ro: readonly

Here any user in the "group_ro" group will be renamed to "readonly".