DEVFS - rollup - Userland, nrelease, and installer

DEVFS - rollup - Userland, nrelease, and installer

* Remove MAKEDEV and MAKEDEV use in the build system.

* Add a new libc function fdevname() taking a file descriptor and
  returning the device name.

* Remove the MFS /dev for the CD and adjust the nrelease build system
  to use devfs.

* Installer adjustments:  PFI search path, remove device node maker
  (it is non-applicable with devfs).

* Add mount_devfs to the build Makefile.

* Change dhclient to use an auto-cloning bpf.

* The VN device now uses labels by default, and no longer supports
  non-label configurations (though you can of course ignore the label
  area and just r/w the raw device anyway).

Submitted-by: Alex Hornung <ahornung@gmail.com>, and Matthew Dillon