proc-cmdline: return proper errors from shall_restore_state()

Let's not eat up errors in shall_restore_state(), but in the consumers
instead, just for the sake of keeping the library calls generic.

src/basic/proc-cmdline.c

@@ -141,5 +141,5 @@ int shall_restore_state(void) {
         if (r == 0)
                 return true;
 
-        return parse_boolean(value) != 0;
+        return parse_boolean(value);
 }