write: doesn't check for tty group

write: doesn't check for tty group

write(1) selects a wrong tty, because there is not a proper
check of tty group ownership:

	$ write kzak
	write: kzak is logged in more than once; writing to tty7
	write: /dev/tty7: Permission denied

	$ ls -la /dev/tty7
	crw--w---- 1 root root 4, 7 2008-07-04 00:32 /dev/tty7
	                  ^^^^

	$ ls -la /usr/bin/write
	-rwxr-sr-x 1 root tty 11864 2008-04-02 16:24 /usr/bin/write
	      ^           ^^^

We have to check for tty group owner, because we don't have
permissions to write to arbitrary tty.

Fixed version:

	$ write kzak
	write: kzak is logged in more than once; writing to pts/6
	                                                    ^^^^
	Message from test@nb on pts/7 at 15:22 ...

	^C

	$ ls -la /dev/pts/6
	crw--w---- 1 kzak tty 136, 6 2008-07-07 15:35 /dev/pts/6
	                  ^^^

Addresses-Red-Hat-Bugzilla: #454252
Signed-off-by: Karel Zak <kzak@redhat.com>