Add an option to emit extra debug code in the generated protocol functions.

Add an option to emit extra debug code in the generated protocol functions.
The debug version of indirect.c has proven very helpful during the
development of new client-side and server-side GLX protocol code.  This is
put here instead of in the DISPATCH and DISPATCH_RETURN macros in dispatch.c
for two reasons.

1. I only want the debug information when indirect-rendering is used.

2. Functions that require a reply from the server, some of which do not have
a return value (e.g., glFlush, glFinish, glGetLightfv, etc.), do not need
the extra glFinish call.