who --mesg (-T) can use a more accurate test for TTY writability

who --mesg (-T) can use a more accurate test for TTY writability

Enabled when coreutils is configured with --with-tty-group.
Based on a patch written by Piotr Gackiewicz.  Details at
http://bugzilla.redhat.com/454261

* src/who.c (is_tty_writable): A new function returning true if a TTY
device is writable by the group.  Additionally it checks the group to be
the same as TTY_GROUP_NAME when compiled with --with-tty-group.
* m4/jm-macros.m4: Introduce a new configure option --with-tty-group.
* NEWS: Mention the change.