src/: Fix internals to carry around address families.

This is preparatory work for IPv6 support, but there isn't any yet.
The code still acts directly on address structures (having asserted that
the family is AF_INET); switching on the address family is for later.

[ No functional change intended in this commit. -iwj ]

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>