c-stack: assume stack overflow if SA_SIGINFO unsupported

c-stack: assume stack overflow if SA_SIGINFO unsupported

* lib/c-stack.c (SIGACTION_WORKS): Rename...
(SIGINFO_WORKS): ...since gnulib module guarantees that (most) of
sigaction will work.
(die): Assume stack overflow if siginfo doesn't work, to let Haiku
behavior match Linux.
* tests/test-c-stack.c (main): Prefer NULL for pointers.

Signed-off-by: Eric Blake <eblake@redhat.com>