sockets: Limit SocketAddressLegacy to external interfaces

SocketAddressLegacy 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.  SocketAddress is the
equivalent flat union.  Convert all users of SocketAddressLegacy to
SocketAddress, except for existing external interfaces.

See also commit fce5d53..9445673 and 85a82e8..c5f1ae3.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1493192202-3184-7-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Minor editing accident fixed, commit message and a comment tweaked]

Signed-off-by: Markus Armbruster <armbru@redhat.com>
