add defun2 for adding documentation to function written by C language

I add defun2 function for adding documentation string to function writtin by C.
~~~
example in eus/c/arith.c
- defun(ctx,"=",mod,NUMEQUAL);                                                                                                 
+ defun2(ctx,"=",mod,NUMEQUAL,"euqal numbers");
~~~

We can take another strategy updating defun function as defun2 like and adding documentation to all existing code.
~~~
defun (ctx, "func", mod, FUNC) -> defun (ctx, "func", mod, FUNC, NULL)
~~~  

related to https://github.com/jsk-ros-pkg/jsk_roseus/issues/178