sockets: Prepare inet_parse() for flattened SocketAddress

sockets: Prepare inet_parse() for flattened SocketAddress

I'm going to flatten SocketAddress: rename SocketAddress to
SocketAddressLegacy, SocketAddressFlat to SocketAddress, eliminate
SocketAddressLegacy except in external interfaces.

inet_parse() returns a newly allocated InetSocketAddress.  Lift the
allocation from inet_parse() into its caller socket_parse() to prepare
for flattening SocketAddress.

Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <1493192202[email protected]>
Reviewed-by: Eric Blake <[email protected]>
[Straightforward rebase]