weston: Use systemd notify,

Using systemd notify fixes the problem with dependency chain in case
if other services depend on running weston.
This change required more robust handling of weston modules arguments
due to custom argument parser impmentation in weston (only last
--modules argument is accepted) and fixes the bug in modules handling
in the weston-start script (only last argument is actually parsed by
weston). Master branch implements systemd-notify thus backport but
doesn't utilize modules anymore so this change is mostly dunfell
specific.

Upstream-status: Backport

Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>
Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>