write: tell when effective gid and tty path group mismatch

write: tell when effective gid and tty path group mismatch

Most commonly this error happens when write(1) executable does not have
correct group ownership and setgid bit.  The earlier message was unclear
what exactly was wrong.

$ mesg
is y
$ write testuser
write: you have write permission turned off

Alternatively the 'getegid() == s.st_gid' could be considered unnecessary.
Afterall if to write to destination tty is denied that does not go unnoticed
at thet time when tty is opened.

Reviewed-by: Benno Schulenberg <bensberg@justemail.net>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>