logind: fix seat_can_tty() to check for VTs

logind: fix seat_can_tty() to check for VTs

A seat provides text-logins if it has VTs. This is always limited to seat0
so the seat_is_seat0() check is correct. However, if VTs are disabled, no
seat provides text-logins so we also need to check for the console-fd.

This was previously:
  return seat_is_vtconsole();
It looked right, but was functionally equivalent to seat_is_seat0(). The
rename of this helper made it more obvious that it is missing the VT test.