nfsv4: consult export file on getRootInode()

NFSv4 do not have mount operation. As a result any host
can establish TCP connection and NFSv41 session. Nevertheless,
we can reject access to root of file system if there are no exports
for the client.

Acked-by: Gerd Behrmann
test: add unit test for FileHandle and FileHandleBuilder
vfs: construct new file handles from old one

convert old pseudoFS handles into new one
vfs: support old file handles

NOTE: this can be 100% correct behavior as it uses
first match export entry for a particular client.
Merge branch 'for-2.2' into 0.5