Warning fixes from Christos Zoulas

- shadowed variables
- signed/unsigned confusion
- const lossage
- incomplete structure initializations
- unused code

lib/sl/sl.h

@@ -41,10 +41,10 @@
 typedef int (*cmd_func)(int, char **);
 
 struct sl_cmd {
-  char *name;
+  const char *name;
   cmd_func func;
-  char *usage;
-  char *help;
+  const char *usage;
+  const char *help;
 };
 
 typedef struct sl_cmd SL_cmd;