[issue1538] mountroot should probe file systems

[Matthew Dillon]

:We already have vfsconf_each() which could then be used in sys/kern/vfs_conf.c to 
:iterate through all available filesystems and call their respective vfs_probe() 
:method until one of them returns no error.
:
:To minimize the impact this might have on the boot time this should only be done 
:if no fs is specified in the vfs.root.mountfrom.
:
:Any thoughts?
:
:Cheers,
:Alex Hornung

    The disklabel also has the filesystem type stored in it for each
    partition.  The mount command can just probe that.  We can add an
    ioctl if necessary.

    I would rather NOT try to auto-probe filesystems since different
    filesystems put their volume header in different places.  For example,
    it is possible for both the UFS and HAMMER volume headers to be present
    (I've done it), though of course only one of the two will actually be
    the correct filesystem.

						-Matt