* Fix a huge gaping hole in nix-env w.r.t. the garbage collector.

  Nix-env failed to call addPermRoot(), which is necessary to safely
  add a new root.  So if nix-env started after and finished before the
  garbage collector, the user environment (plus all other new stuff)
  it built might be garbage collected, leading to a dangling symlink
  chain in ~/.nix-profile...

* Be more explicit if we block on the GC lock ("waiting for the big
  garbage collector lock...").

* Don't loop trying to create a new generation.  It's not necessary
  anymore since profiles are locked nowadays.