rpcgen: add a C code generator for XDR protocol specs

rpcgen: add a C code generator for XDR protocol specs

This implements a C code generator that emits code that is
(almost) identical to the classic 'rpcgen' program. The
key differences are:

 - Skip inlining of calls for struct fields
 - Skip K&R style function prototypes in headers
 - Use int64_t instead of quad_t for OS portability
 - Saner whitespace / indentation

The tests/demo.c and tests/demo.h files were created using
the traditional 'rpcgen' program, and then editted to cut
out the leading boilerplate, and the differences mentioned
above.

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