SUNRPC: Simplify rpcb_register() API

Bruce suggested there's no need to expose the difference between an error
sending the PMAP_SET request and an error reply from the portmapper to
rpcb_register's callers.  The user space equivalent of rpcb_register() is
pmap_set(3), which returns a bool_t : either the PMAP set worked, or it
didn't.  Simple.

So let's remove the "*okay" argument from rpcb_register() and
rpcb_v4_register(), and simply return an error if any part of the call
didn't work.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>