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>