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>