Generate strings files for localizing PPD options (Issue #5194)

Generate strings files for localizing PPD options (Issue #5194)

cups/language-private.h:
- New _cupsMessageAdd and _cupsMessageSave private APIs.

cups/language.c:
- Implement new _cupsMessageAdd and _cupsMessageSave private APIs.

cups/ppd-cache.c:
- Generate strings array when loading cache from PPD.
- Remove strings_uri (just pulling strings from PPD now).

cups/ppd-private.h:
- Remove strings_uri and add strings array to cache.

scheduler/client.c:
- Add support for /strings/NAME.strings
- Cleanup implementation of GET/HEAD/POST to files.

scheduler/ipp.c:
- Return local strings file URI.
- Clean up copy_printer_attrs implementation.

scheduler/printers.c:
- Save strings array to cache file, drop support for strings_uri.

scheduler/printers.h:
- Add strings filename to cupsd_printer_t structure.