lsusb: Support non-ASCII string descriptors.

This replaces the get_string function with get_dev_string, which
obtains the string descriptor in raw UTF-16 and attempts to iconv
it to the current locale's encoding. If that fails, or if the system
lacks the required functions, it falls back to ASCII.
Merge pull request #14 from SpecLad/master

Support non-ASCII string descriptors in lsusb