afsocket: Restore localport() support for network destinations

A while ago, the functionality to set the local, originating ports of
network destinations has been lost. This patch restores that
functionality by setting the port on the socket address when it is being
set up.

Reported-by: Sebastiaan Hoogeveen <zebaz@xs4all.nl>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>