unix: uClibc doesn't like NULL as a buffer arg to realpath().

So, allocate one explicitly.
modsocket: Add call to freeaddrinfo().
modsocket: Workaround uClibc issue with numeric port for getaddrinfo().

It sucks to workaround this on uPy side, but upgrading not upgradable
embedded systems sucks even more.
unix: Don't error out on #warning directive.