text-based mount.nfs: sort between permanent and temporary errors

The text-based mount.nfs program must distinguish between different types
of errors returned from the kernel.  Permanent errors, like bad mount
options, should cause an immediate failure.  Temporary errors, such as a
connection timeout, should result in a retry of some type.

Add a function that sorts between the two types of errors.  The list of
permanent errors can be adjusted later if needed.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>