new GEOM feature - geom_vol_msdosfs

[Norikatsu Shigemura]

	I made a geom_vol_msdosfs GEOM module.  It provides a feature
	like geom_vol_ffs.  Its own targets are USB memories, USB
	Digital Cameras, and USB/IEEE1394 strorages formated MS-DOS
	(FAT16/FAT32) File System as typical embeded filesystem.

	By using it, we can get a fixed mount pointing like
	/dev/vol/VOLUMELABEL.  So we can write following to /etc/fstab.

/dev/vol/VOLUMELABEL /mnt/VOLUMELABEL msdosfs rw,noauto 0 0


	I tested my md1 (newfs_msdos -L -f 1440), FAT32 formated by
	WindowsXP, and XOSL (Extended Operationg System Loader)
	Dedicated partition(slice?).  Sorry, I have no external 
	eripheral equipments.  Please test in these(avobe targets)
	environments.  And I hope to commit to FreeBSD source tree:-).