rpcgen: define entrypoint for running new rpcgen impl

rpcgen: define entrypoint for running new rpcgen impl

The new program takes the form

  rpcgen [--mode source|header|repr] \
         [--header include] \
         xdr-file output-file

If '--mode' is not given it parses the XDR file but does not
generate anything, which is useful as a syntax check. The
'source' mode gives the '.c' file content, while 'header'
gives the '.h' file content. 'repr' gives a representation
of the abstract syntax tree, mostly useful for debugging
the parser.

If '--header' is given, it is added as a local #include ".."
statement in the output and is valid for either 'header'
or 'source' modes.

Either 'xdr-file' or 'output-file' can be omitted in which
case they default to stdin/stdout respectively.

This rpcgen program will directly include the 'config.h'
header in its output.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>