SUNRPC: Clean up RPCBIND_MAXUADDRLEN definitions

Clean up: Replace the single-integer definition of RPCBIND_MAXUADDRLEN
with a definition that is based on previously defined address string
sizes, and document the way this maximum is calculated.  Also provide
a separate macro for the size of the port number extension.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>