lib: a few more trivial changes for C++ compatibility

lib: a few more trivial changes for C++ compatibility

* command_graph.h: stop using "new" as a parameter name as that's a
  reserved C++ keyword.

* module.h: avoid using C99 designated initializers since C++ doesn't
  support them. This change hurts code readability quite considerably,
  so we should try to find a better solution later.

* pw.h: remove unneeded empty structure to silence a C++ warning.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>