Fix string literals in fprintf and friends

Both these are from [-Werror=format-security]
sig.c:262:5: error: format not a string literal and no format arguments
global.c:517:3: error: format not a string literal and no format arguments

ps/global.c

@@ -514,6 +514,6 @@ catastrophic_failure(const char *filename,
 		     unsigned int linenum,
 		     const char *message)
 {
-  error_at_line(0, 0, filename, linenum, message);
+  error_at_line(0, 0, filename, linenum, "%s", message);
   exit(EXIT_FAILURE);
 }