src/sulogin.c: Make static variables local to main()

src/sulogin.c: Make static variables local to main()

Those variables are only used in main().  Restrict their scope.
Keep them static (.bss), as changing that may be dangerous.

Suggested-by: Samanta Navarro <ferivoz@riseup.net>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>