Notes on DEVFS serial number support

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

    Note:  With these commits a full world & kernel build is required to
    get the new features.  Multi-volume HAMMER mounts specifying devtab
    labels won't work yet (I'm not sure how best to parse them), but
    they should work using serno/* paths.

    My recommendation here is that people not worry so much about mounting
    by serial number unless they have a bunch of drives.  Ultimately this
    will become a very powerful feature as networked drive support is added.
    For now its claim to fame is to remove issues with drive unit iterations
    when a drive is removed.

    Also note that vinum is not operational in master at the moment.

    --

    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.s1f      /fubar     hammer  rw       1       1
    
       /etc/devtab:
    
        driveA          serno   L41JYE0G

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>