Teach gadget/usbstring to expect UTF-8 strings, not ISO-8859/1 ones.
This just gets rid of an API issue:  no hacks needed for non-Western
languages, and multi-language support will be lots easier.

Current drivers won't notice the API change, they use US-ASCII (which
is a strict superset of both encodings).

Future drivers may want to teach utf8_to_utf16le() about the four-byte
encodings, so they can emit surrogate pairs for those Unicode characters.