sys/socket.h: reconcile bsd and osv variants

Some structures are duplicated; deduplicate them.

A few are source-compatible but not binary-compatible; use the ones from
<bits/socket.h>.

Others are both source- and binary- compatible; put them in a new header
<sys/__socket.h> which is included from both.

Work around a problem with the byteorder functions/macros.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Tested-By: Benoit Canet <benoit@irqsave.net>