Allow to choose launcher and disable systemd-logind at run-time

Trying to start Weston as quickly as possible at boot-up, it looks like on my systems (Debian Sid/unstable with Weston 8), that due to the slow start of systemd-logind (900 ms according to systemd-analyze blame), Weston’s start-up is delayed by by over 1.8 seconds due to:

https://gitlab.freedesktop.org/wayland/weston/-/blob/master/libweston/launcher-logind.c#L97-98

I couldn’t find a switch to choose the launcher at run-time, or to disable systemd-logind support at run-time. It would be nice to have, so that distributions packages could be used.