Install: name what waits for a restart; uninstall survives a failed Nix uninstall

A fresh install into a running session left the keyboard layout and the
SteamVR dashboard patches (VR pet, the VR keyboard's suggestion strip, ...)
silently off until a reboot: gamescope reads XKB_DEFAULT_* only at its
start, and SteamVR opens its DevTools port (VRWebHelper.DebuggerEnabled)
only at its start. Neither can be applied at runtime.

- install.sh restart-check (run on every switch by session.nix, and by
  install at its end): compares the keyboard-layout drop-in with the
  running gamescope session's environment and checks whether SteamVR runs
  without port 8087; install ends with "Restart once" listing them.
- uninstall: a failing nix-installer uninstall (e.g. /nix busy) no longer
  aborts the script before the ~/.config/home-manager link and Home
  Manager state are removed; it says to reboot and run uninstall again.
- cleanup check: restart-check on fake cgroup/proc dirs.

docs/steamvr-debugger.md

@@ -21,8 +21,11 @@ to its previous value when SteamVR stops, also after a rollback or uninstall
 (without Nix). A value you set to `true` yourself is never touched.
 
 **The first time, restart SteamVR once** (e.g. reboot); until then the
-dashboard patches wait. Turned off, the setting is restored at the switch
-(or when SteamVR stops, if it runs).
+dashboard patches wait. SteamVR opens the port only when it starts, so no
+switch or install can do it for a running SteamVR; the switch (and
+`install.sh install` at its end) says so while SteamVR runs without the port
+(`install.sh restart-check`). Turned off, the setting is restored at the
+switch (or when SteamVR stops, if it runs).
 
 ## Security