sulogin: reduce vulnerability surface

sulogin: reduce vulnerability surface

Hopefully these changes are unreachable code, but better safe than sorry
when dealing with setuid root code that is installed everywhere.  Quite
obviously the introduced abort() calls protect from impossible inputs.

Secondly set all possible data to be read-only in attempt to make it more
difficult to alter anything at all.

Reference: https://www.securecoding.cert.org/confluence/display/c/DCL00-C.+Const-qualify+immutable+objects
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>