text-based mount.nfs: Fix po_rightmost() enum return values

Neil observed that po_rightmost() now returns enum values from both

enum {
	PO_NOT_FOUND = 0,
	PO_FOUND = 1,
}

and

enum {
	PO_KEY2_RIGHTMOST = 1,
	PO_KEY1_RIGHTMOST = -1,
}

It would be cleaner to use a single enum for po_rightmost()'s return value.

We take the next logical step and create specific types for the return
values in order to ensure we don't mix the enum values, and to document
explicitly what return values callers can expect.

This could have been a simpler patch, but I think the end result is a
cleaner overall parser API.

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