Add docs for arpa/inet.h, allow testing deprecated functions

This adds docstrings for arpa/inet.h, having traced down the appropriate specs and possible deprecation for each function. In addition, the function parameters have been aligned with POSIX.

Also, the build initially failed due to the use of now-deprecated functions in the test code being treated as hard errors. I've added the -Wno-deprecated-declarations option to the Makefile so these and other deprecated libc functions may be tested.