Add a few missing includes

A few files were missing includes, leading to compile-time warnings, add
them.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
afsocket: Move the SOL_IP and SOL_IPV6 stuff to where they're used

Move the SOL_IP and SOL_IPV6 redefinitions from afinet.c to
socket-options-inet.c, where they're used.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>