src/addrfam.c, src/...: Abstract out address-family-specific details.

Functions which know about the internals of particular address types and
socket-address structures live in a new file `addrfam.c'.

Some details aren't handled yet: notably, we don't have conversions
between internal address structures and external formats such as text
strings or reverse-lookup domain names.

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