The file system side of the vop_link and vop_remove interfaces have
changed.

1) The first two arguments to vop_link have been renamed, to `dvp' and
`vp', respectively.  The old names were wrong and very confusing.

2) It is now the responsibility of the file system to reject links and
unlinks of directories, if it so desires.


Note to John: This will probably require (minor) source updates to the
AFS module, though existing modules should continue to work okay.