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/development.md

@@ -47,8 +47,8 @@ scripts/
 modules/
   session.nix                  switch: outer bus/runtime dir, user services on switch
   cleanup.nix                  switch: `cleanup --orphans`; steam-frame-nix-cleanup on PATH
-  cleanup/package.nix          build: install.sh as a command (cleanup, steamvr-debugger-arm)
-  cleanup/check.nix            test: install.sh cleanup on fake home/runtime dirs
+  cleanup/package.nix          build: install.sh as a command (cleanup, steamvr-debugger-arm, restart-check)
+  cleanup/check.nix            test: install.sh cleanup and restart-check on fake home/runtime dirs
   portal.nix                   Steam session portal config (session.portalFix)
   portal/check.nix             test: KDE FileChooser on by default, absent when disabled
   applications-menu.nix        applications.menu link for KDE apps (session.applicationsMenu)