Pre-register server side RPCSEC GSS support

When --enable-gss is specified on the ./configure command line,
have the library automatically register server-side support for the
RPCSEC_GSS auth flavor.

The complication is that specific interaction is required with the
RPC client if GSS authentication fails. GSS authentication sometimes
has to squelch the normal reply done by svc_getreq(), and substitute
its own.

_svcauth_gss() already has a boolean argument to do this.  But
_authenticate() is an official API (see rpc/svc_auth.h). We can't
alter its synopsis.

Instead of adding a "no_dispatch" argument to our existing
_authenticate() API, preserve its synopsis for backwards
compatibility, and introduce a second external authentication API
for the dispatcher.

This matches a similar API change done in the Solaris libtirpc.

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