DEVTAB - Add mountroot & fstab support for serial numbers, and devtab.

DEVTAB - Add mountroot & fstab support for serial numbers, and devtab.

* The vfs.root.mountfrom /boot/loader.conf variable may now specify
  devfs aliases, allowing it to specify root mounts by serial number.
  Here is an example:

    vfs.root.mountfrom="hammer:serno/L41JYE0G.s1d"

  Note that vfs.root.mountfrom may NOT currently specify /etc/devtab labels.

* /etc/fstab may now specify devfs relative paths (such as serial numbers)
  as well as /etc/devtab labels.  A ".suffix" may be used to extend the
  translation so /etc/devtab only needs to specify the base label.

  Example /etc/fstab:

    # Device                Mountpoint FStype  Options  Dump    Pass
    #
    driveA.s1d              /          hammer  rw       1       1
    driveA.s1b              none       swap    sw       0       0
    driveA.s1a              /boot      ufs     rw       1       1
    serno/L41JYE0G.s1d      /fubar     hammer  rw       1       1

   /etc/devtab:

    driveA          serno   L41JYE0G