Provide ntohl, ntohs, htonl and htons no-op macros on big endian systems.

Provide ntohl, ntohs, htonl and htons no-op macros on big endian systems.

This means that there are always macros or functions for ntohl, ntohs,
htonl and htons available, so eliminate use of HAS_NTOHL etc, and
unconditionally compile the code that it was protecting. However, as code
on CPAN is using these guard macros, define all of them in perl.h

(Technically the 4 are not quite no-ops, as they truncate their values to
32 or 16 bits, to be consistent with the implementations for platforms which
need re-ordering.)