Fix most warnings when compiling in pedantic mode.

The code should compile without warnings coming from the rsp plugin itself in pedantic mode.
There are still some warnings from m64p_types.h (// coments, unammed struct/union).

If I want to go further in warnings suppression, I need to know for which standard we code (ie what option should I give to gcc -std=c89, c90, c99, ???).