weston: wrapper for weston modules argument

Due to custom option parser implementation weston accepts only one
argument of a given type. As the result if multiple modules add
multiple --modules agruments only last will be used. This fix
introduces wrapper around modules in terms of weston-init to prepare
proper modules argument for weston and adds systemd-notify module into
weston-init module to support Type=notify in systemd service file.

Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>