mount.nfs: add EX_SUCCESS exit code

mount.nfs: add EX_SUCCESS exit code

We've had some recent trouble, especially in the umount code, that appears
to be due to functions returning a 1 or a 0 return code when they should be
returning a mount exit code (such as EX_FAIL) or a 0.

To help clearly distinguish these two classes of functions, define an
EX_SUCCESS exit code, which is equal to zero.

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