do not rely on obsolete feature

function prototype with unspecified argument is obsolete since c99.

additionally some of these function which don't take any argument were
being called with a `const char *` arg, which is UB.

fix both these issues by declararing ALL the components to accept a
`const char *`, and name the arg "unused" if it's meant to be ignored.

slstatus.c

@@ -12,7 +12,7 @@
 #include "util.h"
 
 struct arg {
-	const char *(*func)();
+	const char *(*func)(const char *);
 	const char *fmt;
 	const char *args;
 };