sockets: New helper socket_address_crumple()

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 <[email protected]>
Message-id: 1490895797[email protected]
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Max Reitz <[email protected]>