The _next method shouldn't take a path pointer (and a path_len
pointer) as 100% of current users use the full path and ignore
the filename.

Plus let's add some docs and a unit test.

This is what it's meant all along, but now we actually have multiple
implementations, it's clearer to use the name of the library.

As an alternative to OpenSSL when we're on OS X. This one can actually
take advantage of stacking the streams.

Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Have the ALLOC_OVERFLOW testing macros also simply set_oom in the
case where a computation would overflow, so that callers don't
need to.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.