Give a proper diagnostic for e.g., `fmt -c -72'.

tests/fmt/basic

@@ -34,6 +34,10 @@ my @Tests =
      ['no-file', 'no-such-file',
       {ERR => "fmt: cannot open `no-such-file' for reading:"
        . " No such file or directory\n"}, {EXIT => 1}],
+     ['obs-1', '-c -72',
+      {ERR => "fmt: invalid option -- 7; -NUMBER is recognized only when it"
+       . " is the first\noption; use -w N instead\n"
+       . "Try `fmt --help' for more information.\n" }, {EXIT => 1}],
     );
 
 my $save_temps = $ENV{DEBUG};