mount command: Replace clnt_ping() and getport() calls in probe_port()

Update the mount command's probe_port() function to call the new shared
rpcbind query and RPC ping functions.  This provides immediate support
for
rpcbind v3/v4 queries, and paves the way for supporting AF_INET6 in the
probe_bothports() path.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>