Modify vfs_register() to use a hash calculation

Modify vfs_register() to use a hash calculation
on vfc_name to set vfc_typenum, so that vfc_typenum doesn't
change when file systems are loaded in different orders. This
keeps NFS file handles from changing, for file systems that
use vfc_typenum in their fsid. This change is controlled via
a loader.conf variable called vfs.typenumhash, since vfc_typenum
will change once when this is enabled. It defaults to 1 for
9.0, but will default to 0 when MFC'd to stable/8.

Tested by:	hrs
Reviewed by:	jhb, pjd (earlier version)
Approved by:	re (kib)
MFC after:	1 month