[new/wip] greetd/gtkgreet

hi,

third attempt at a port for https://git.sr.ht/~kennylevinsen/gtkgreet &
https://git.sr.ht/~kennylevinsen/greetd, i'm at a point where i have
something that 'works', eg:

- greetd is started as root via rc.d
- it spawns cage -- gtkgreet (still as root, but should technically be
with a dedicated user) to show the graphical login manager in a caged
wayland session
- when the user logs in, the cage wayland session exits, and the chosen
wayland session (eg labwc, sway, wayfire for now on OpenBSD, list to
fill in /etc/greetd/environments) is started as the given user.
- exiting that session (eg pkill labwc), the greeter is shown again.

things to improve:
- properly call login_fbtab instead of the super ugly chown i'm doing
(see patches/patch-greetd_src_session_worker_rs, i know nothing to
rust, and so far i hate this first experience with it)

- do something to mkdir ~/.local/run with the to-be-logged-in user ?

- see if it can work on other ttys than ttyC0 (but then fbtab should be
amended ?)

- see if /var/run/greetd should be created/deleted outside of the rc
script and in the rust code ? this one is used only for the 'greeter'
wayland session

- see how it can work with a dedicated unpriv user

feedback on the rust patches, the rc script, the port & testing much
welcome :)

the other greeters (regreet,qtgreet) might be ported too.

Landry