sulogin: use common password-checking routine.

sulogin: use common password-checking routine.

This needed some extensions correct_passwd() function,
which got renamed ask_and_check_password() to better describe what it does.

function                                             old     new   delta
ask_and_check_password_extended                        -     215    +215
ask_and_check_password                                 -      12     +12
vlock_main                                           394     397      +3
sulogin_main                                         494     326    -168
correct_password                                     207       -    -207
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 1/1 up/down: 230/-375)         Total: -145 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>