sockets: New helper socket_address_crumple()

SocketAddress is a simple union, and simple unions are awkward: they
have their variant members wrapped in a "data" object on the wire, and
require additional indirections in C.  I intend to limit its use to
existing external interfaces.  New ones should use SocketAddressFlat.
I further intend to convert all internal interfaces to
SocketAddressFlat.  This helper should go away then.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1490895797-29094-8-git-send-email-armbru@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
