ssp: knock out some trivial warnings that come up with WARNS=6
A future commit will rebuild this as part of libssp. The exact warnings are
fairly trivially fixed:
- No previous declaration for __stack_chk_guard
- idx is the wrong type, nitems yields a size_t
- Casting away volatile on the tmp_stack_chk_guard directly is a no-no.
Reviewed by: kib, emaste, pfg, Oliver Pinter (earlier version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22943