Add a feature to support supplementary groups

[Jaime Sicam]

The limitation of the PAM plugin for proxy users is that a lookup for a group is limited to the initial group and not its supplementary groups as discussed in https://bugs.launchpad.net/percona-pam-for-mysql/+bug/1091566

Testing PAM authentication with initial and supplementary groups

Create proxy user:
mysql> create user ''@'' identified with auth_pam as 'mysqld,developer=developer_user';

Note:
mysqld => PAM
''@'' => Proxy user
developer => Unix Group
developer_user => Proxied User

Create proxied user:
mysql> create user developer_user@localhost identified by 'sample_password';

Configure proxy:
mysql> grant proxy on developer_user@localhost to ''@'';
mysql> FLUSH PRIVILEGES;

Create group:
#groupadd developer;

Create users:
#useradd -g developer devuser1
#passwd devuser1
#useradd devuser2
#passwd devuser2

#usermod -G developer devuser2

Testing access:
#mysql -u devuser1 -p
mysql> select user(), current_user(), @@proxy_user;
+--------------------+--------------------------+--------------+
| user() | current_user() | @@proxy_user |
+--------------------+--------------------------+--------------+
| devuser1@localhost | developer_user@localhost | ''@'' |
+--------------------+--------------------------+--------------+

#mysql -u devuser2 -p
mysql> select user(), current_user(), @@proxy_user;
+--------------------+----------------+--------------+
| user() | current_user() | @@proxy_user |
+--------------------+----------------+--------------+
| devuser2@localhost | @ | NULL |
+--------------------+----------------+--------------+

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-97