Uninstall: wait until nothing uses /nix before removing Nix

nix-installer uninstall failed at `systemctl stop nix.mount` ("Job
failed") while programs started from the Nix store still ran: a terminal
session's tools, or apps that mapped Home Manager's mime.cache.

- Before nix-installer runs, after Home Manager is gone, uninstall scans
  /proc for the user's processes whose exe, cwd, root, an fd or a mapped
  file is in /nix, and lists them by name and PID (the shells it was
  started from included; itself, its subshells and its curl | bash
  pipeline skipped). Close them and press Enter to re-check, or abort;
  with --yes or without a terminal it stops: "close these or reboot, then
  run uninstall again". Nothing is killed (replaces the old kill prompt).
- nix-installer runs from a copy outside /nix.
- cleanup check H: a fake /proc (STEAM_FRAME_NIX_PROC) with programs from
  the store stops before Nix; without them Nix is removed.

docs/development.md

@@ -49,7 +49,7 @@ modules/
   cleanup.nix                  switch: `cleanup --orphans`; steam-frame-nix-cleanup on PATH
   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,
-                               install --clone against local bare repos
+                               install --clone against local bare repos, uninstall on a fake /proc
   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)