wall: add --group option

The wall command on AIX supports a "-g" option to limit the message
to a group of users by gid.  Add compatibility to the Linux version.

Thanks to Sami Kerola <kerolasa@iki.fi> for an initial skeleton
implementation.

[kzak@redhat.com: - rename max to ngroups
                  - add free_group_workspace()
                  - some cosmetic changes]

Reference: http://www.ibm.com/support/knowledgecenter/ssw_aix_61/com.ibm.aix.cmds6/wall.htm
Signed-off-by: Jim Patterson <jimp@wegrok.net>
Signed-off-by: Karel Zak <kzak@redhat.com>