Mark the return value from strerror as a constant

* src/emacs.c (main): Mark the return from strerror as a constant,
since it shouldn't be changed (bug#43982).

* lib-src/movemail.c (pfatal_and_delete): Ditto.

Copyright-paperwork-exempt: yes

lib-src/movemail.c

@@ -580,7 +580,7 @@ pfatal_with_name (char *name)
 static void
 pfatal_and_delete (char *name)
 {
-  char *s = strerror (errno);
+  const char *s = strerror (errno);
   unlink (name);
   fatal ("%s for %s", s, name);
 }

src/emacs.c

@@ -1273,7 +1273,7 @@ main (int argc, char **argv)
 	      || (fcntl (STDIN_FILENO, F_DUPFD_CLOEXEC, STDOUT_FILENO)
 		  != STDOUT_FILENO))
 	    {
-	      char *errstring = strerror (errno);
+	      const char *errstring = strerror (errno);
 	      fprintf (stderr, "%s: %s: %s\n", argv[0], term, errstring);
 	      exit (EXIT_FAILURE);
 	    }