Use `CLAIM_FH` for NFS:OPEN instead of inefficient `CLAIM_NULL`

[Peter-JanGootzen]

Currently when opening a file CLAIM_NULL is used in conjunction with CURRENT_FH=parent->fh and the filename of the file to be opened. This requires tracking the parents of files plus the filenames of files inside of virtionfs.

In NFS 4.1 there is a CLAIM_FH flag that allows you to open the file that the CURRENT_FH points to. Removing this extra bookkeeping overhead from the client.

Blocked by #2 and #15 because NFS 4.1 is required.

[Peter-JanGootzen]

Implemented in #15