hostinfo: improve detection of linux desktops (#20848)

Our desktop detection was using "wayland-1" as a search string for
detecting wayland desktops, but many desktops use other indexes for the
session. Additionally we did not detect mir or gamescope.

Add a new detection method that leans on systemd-logind (if available)
and fall back to using the existing method of looking for open unix
sockets, but add search strings and tail the index off of the wayland
session detection.

Fixes #20847

Signed-off-by: Claus Lensbøl <claus@tailscale.com>