This will help programs that need to URL-escape strings.
Also, this changes `do_hvis()` to not fallback on `do_svis()` for chars in
`extra` -- that `do_hvis()` was doing that seems like an oversight. Christos
Zoulas, of NetBSD, agrees. `do_hvis()` still falls back on `do_svis()` for
characters not in the RFC 1808 / 3986 to-be-escaped set *and* characters not in
the `extra` set -- that much seems to have been the intent.
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