accessing files which it shouldn't be able to. This required a better
approximation of VOP_ACCESS for NFSv2 (NFSv3 already has an ACCESS rpc
which is a better solution) and adding a call to VOP_ACCESS from VOP_LOOKUP.
PR: kern/876, kern/2635
Submitted by: David Malone <dwmalone@maths.tcd.ie> (for kern/2635)