webui, htsbuf: Content-Disposition escape chars are not correct.

webui, htsbuf: Content-Disposition escape chars are not correct.

When attempting to download a recording with a comma Google Chrome will
fail with ERR_RESPONSE_HEADERS_MULTIPLE_CONTENT_DISPOSITION. This is
because the comma ',' in the filename*=UTF-8'' field was not escaped.

This commit implements the defined list of non-escape characters from
RFC8187 based on htsbuf_append_and_escape_url.

The same problem occurs in issue #2086. Fixed in 2fdfe4836 "webui: fix the
attachment; filename encoding, fixes #2086" and broken again in ab9fc249a
"fix htsbuf_append_and_escape_url() - don't escape more allowed characters,
fixes #3721".

Only tested in release/4.2.

https://bugs.chromium.org/p/chromium/issues/detail?id=454165