Treat passed strings as raw-text when percent-escaping in epg

Treat passed strings as raw-text when percent-escaping in epg

The strings contained in gpg keys can contain UTF-8 data, but can also
use percent-escapes to encode non-ASCII chars.  When converting those
escapes, use 'raw-text' coding system rather than 'string-to-unibyte',
since the latter signals an error for non-ASCII characters.

* lisp/epg.el (epg--decode-percent-escape): Convert the passed
string to raw-text before treating percent escapes (Bug#38512).