Filenames with spaces; output beside the input file; warning fixes (from dev c08eb1bbfa4c)

- Filenames may contain spaces: quote on the command line; filename
  lists use a standalone "/" separator; unseparated names that do not
  exist are rejoined into names that do (announced); leading ~ expands.
- Without -o, output is written to the input file's directory; -o fully
  specifies the output directory and basename.
- "Word not parsed" warnings now appear without -v, only for text that
  survives removal, with correct line numbers.
- New tst/filename_test.sh regression suite.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

mac/command.cpp

@@ -73,7 +73,9 @@ parse_args(
         output_dir = file_directory(output_basename);
         output_basename = file_basename(output_basename);
     } else if (!input_filenames.empty()) {
-        output_dir = "";  // defaults to cwd via absolute_pathname below
+        // No -o: output goes next to the input file ("" -- an input with no
+        // directory component -- resolves to cwd below).
+        output_dir = file_directory(input_filenames[0]);
         output_basename = file_basename(input_filenames[0]);
     }
     output_dir = absolute_pathname(output_dir);