installer: Fix the "missing /var/run/sem" issues better.

installer: Fix the "missing /var/run/sem" issues better.

* Add -C to the tmpfs mount of /var/run on the USB image. This
  ensures that the USB image itself keeps the ppp/, sem/, and
  wpa_supplicant/ dirs under /var/run.

* Since we do not cpdup /var/run explicitly during installation
  it ends up empty on the installed system. Ensure that it has
  the subdirectories it needs by running mtree on /var.

* It looks like the 1e5a476188185cf1161f4fe3ffc060465562ef0d
  change could be reverted because the issue never was that
  /var/run/sem wasn't existing _after_ make upgrade. The mtree
  in the preceding installworld takes care of it. It doesn't
  hurt either, so just shuffle the commands a bit downwards.