server: Add a socket with an existing fd

This adds functionality to allow system-level control over handing out
file descriptors for sockets, to allow tighter security when running a
Wayland compositor under a Wayland session server.  Allows writing
socket activated Wayland servers.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Sung-Jin Park <sj76.park@samsung.com>
Cc: Sangjin Lee <lsj119@samsung.com>