a bit more -Werror compatability

for clang, fix warnings in drivers/, lib/, servers/, sys/, common/.
by turning off fatal warnings (takes effect if the default is on),
fixing warnings or reducing the warning level.

Change-Id: Ia1b4bc877c879ba783158081b59aa6ebb021a50f

include/minix/optset.h

@@ -19,7 +19,7 @@ typedef enum {
  * 'os_val' is then a base passed to strtol().
  */
 struct optset {
-  char *os_name;
+  const char *os_name;
   optset_type os_type;
   void *os_ptr;
   int os_val;