Allow running as root when not using setuid bit

[jbeich]

For debugging (bypass DRM_AUTH), recovery (single user mode) and in some embedded scenarios (root shell instead of sudo/doas) it maybe desirable to run the compositor (and everything else within) as root. Currently, this is not supported to prevent foot-shooting when using builtin server in libseat.

$ sudo sway -c /dev/null
00:00:00.090 [sway/main.c:166] Unable to drop root (we shouldn't be able to restore it after setuid), refusing to start

Similar to #5669 but setuid vs. root is detected automatically. This makes Sway behave like wlroots compositors that don't drop priveleges (e.g., river, phoc, gamescope). Alternatively, drop_permissions() can be removed in favor of seatd-launch(1).

[emersion]

Yeah, now that we have seatd-launch I think it would make sense to just drop all of that suid logic. @kennylevinsen thoughts?

[kennylevinsen]

My main worry would be users SUID'ing sway right now to use the direct/built-in backend, who would experience the change as having sway keep running as full root without as much as a warning. Principle of least surprise and all that.

If we're comfortable with just warning about it in the release notes, then I'd suggest killing it. Maybe wait a little for 0.6.1 rollout though.

[emersion]

Hm right, the transition period is a bit dangerous indeed. Maybe a gradual migration would be better? e.g. error out if we're suid root for a release or two, then drop the logic completely?

[Hjdskes]

Linking this related commit here for posterity: e1db1f8 (from #6756)

[emersion]

Superseded by #7044