login: attempt to run if it has no read/write access to its terminal

login: attempt to run if it has no read/write access to its terminal

If you manage to exec login with a userid other than root, and its
input / output directed to a terminal for which it does not have
read/write access, it will attempt to proceed (and can potentially
hang forever -- but this hang has been fixed in a previous commit).

It's better to check if we have permissions for terminal rather than
do any useless things.

From: Jason Vas Dias <jvdias@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>