cgo: supports c/go callback funcs

cl/_testgo/cgofull/foo.c

@@ -1,6 +1,12 @@
 #include <stdio.h>
 #include "foo.h"
 
-void print_foo(Foo* f) {
+void print_foo(Foo *f)
+{
 	printf("print_foo: %d\n", f->a);
 }
+
+int c_callback(int i)
+{
+	return i + 1;
+}