libnfs: Give correct access() results for directories

Map ACCESS3_{MODIFY,EXTEND,DELETE} to W_OK and ACCESS3_{LOOKUP,EXECUTE}
to X_OK so that nfs_access() gives sensible results for directories.

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>
libnfs: Add access2()

Add nfs_access2(), like nfs_access() but it returns the individual
statuses of R_OK, W_OK and X_OK rather than a single success or failure
status.  This saves the latency and overhead of multiple lookups if an
application tries to determine the status of each of R_OK, W_OK and
X_OK.

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>
Merge pull request #100 from rosslagerwall/access-improve

access() improvements