mesg: remove ability to compile with fchmod(S_IWOTH)

mesg: remove ability to compile with fchmod(S_IWOTH)

The default is to use mesg(1) to modify write access for the "tty"
group, but there is an obscure legacy. If mesg(1) is compiled with the
option "--disable-use-tty-group", then it defaults to using
fchmod(S_IWGRP | S_IWOTH). This means that your tty is then writable
for everyone. Let's get rid of this ugly feature.

Reported-by: Chris Hofstaedtler <zeha@debian.org>
Signed-off-by: Karel Zak <kzak@redhat.com>