proto.h: Use __attribute__nonnull__ if available

proto.h contains a generated PERL_ARGS_ASSERT macro for every function.
It asserts that each parameter that isn't allowed to be NULL actually
isn't.

These asserts are disabled when not DEBUGGING.  But many compilers allow
a compile-time assertion to be made for this situation, so we can add an
extra measure of protection for free.  And this gives hints to the
compiler for optimizations when the asserts() aren't there.

Author: khwilliamson