login: check that after tty reopen we still work with a terminal

login: check that after tty reopen we still work with a terminal

 * the login code assumes that stdin is a terminal, it's better to
   check (by isatty()) that after tty reopen we still have a terminal

 * this patch also removes very old obscure fallback for situations where
   ttyname() returns nothing (then ttyn = "/dev/tty??"). I guess that the
   fake string was originally for utmp records or so. Currently (in last 10
   years...) code requires that the tty name is a real open-able file.
   It means the fake tty name is completely useless.

Reported-by: Yann Droneaud <yann@droneaud.fr>
Signed-off-by: Karel Zak <kzak@redhat.com>