sulogin: allow system maintenance login if root password is empty

sulogin: allow system maintenance login if root password is empty

The current password checking is unable to distinguish between the user
entering an empty password or pressing Control-D. As a result, an empty
password always results in normal startup.

We modify bb_ask to return NULL if Control-D is pressed without entering
a password. The sulogin applet is then modified to only proceed to
normal startup if bb_ask returns NULL. This covers EOF with no password,
interrupt by timeout and ^C.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>