systemd: take over sockets created by systemd

Systemd provides a feature of socket-based activation, details in [1]
This commit adds an implementation to check if sockets were provided by
systemd and adds this as an additional socket to wayland display.
before adding sockets are checked for the correctness:
only AF_UNIX of type SOCK_STREAM are accepted

This is usefull for early rendering use-cases where weston and
early-rendering-application can be started parallel.

[1] https://www.freedesktop.org/software/systemd/man/systemd.socket.html



Signed-off-by: Eugen Friedrich <efriedrich@de.adit-jv.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>