nfs-utils: replace function-specific switches with HAVE_LIBTIRPC

Instead of switching in TI-RPC-specific logic with a function-specific
switch like HAVE_CLNT_VG_CREATE, let's use the more generic
HAVE_LIBTIRPC macro everywhere.

This simplifies ./configure (always a good thing), and makes it more
clear in the source code exactly what the extra conditionally compiled
code is for.

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