systemd-sysext: introduce a global config (#38250)

This PR implements what is proposed in
https://github.com/systemd/systemd/issues/37992.

Having a global config file that supports the same cmdline options for
sysext/confext allows the user to customize the behavior of
systemd-sysext.service unit too, without the need of hacking the service
manually.

The global config will live in
`CONF_PATHS_STRV()/systemd/{sysext/confext}.conf` and it will be
overridden by cmdline, so it is possible to customize a run if
`systemd-sysext` is executed manually.

For now support `--mutable=` (`Mutable`) and `--image-policy=`
(`ImagePolicy`).

diff --cc man/rules/meson.build
Simple merge

diff --cc src/sysext/sysext.c
Simple merge