Once a ypserv is discovered we know the remote UDP port number, but not

Once a ypserv is discovered we know the remote UDP port number, but not
the TCP port number.  Immediately reach out (syncronously via pmap_getport,
to ask the portmap on the ypserv machine) to learn the TCP port number.
Append this as 2 bytes (network byte order) to the binding file, because
an upcoming change will require this information also.
with jmatthew, also ok claudio miod