linux.cf: add libtirpc support. Disabled by default.

linux.cf: add libtirpc support.  Disabled by default.

Currently on Linux, you must run rpcbind in insecure mode (-i) in
order for ttsession to register with rpcbind (the portmapper).  This
is because, on most systems, libc contains an older sun-based rpc
library embedded within it.  The sun-rpc code does not know how to
authenticate with rpcbind when run as a non-privileged user.

Using libtirpc, ttsession can register with rpcbind without requiring
it to be run in insecure mode.  You must have the libtirpc-dev, or
equivalent package installed to use it.

If you want to try this:

- install libtirpc-dev or equivalent
- if your rpcbind process is already running in insecure mode (-i
  option), remove that option and restart it.
- edit config/host.def, (create if it doesn't exist) and add:

  #define HasTIRPCLib  YES

- rebuild CDE (make World).  It's probably a good idea to remove
  /usr/dt/* beforehand to avoid contamination.

Some linux systems seem to incorporate tirpc directly into libc (as
all of the BSD's do AFAIK) so this may not be needed.

I know that at least on Ubuntu systems defining HasTIRPCLib to YES is
required in order to run rpcbind without -i.