Refuse an image URL whose host is not a host name or IP address (closes #275)

A ? written into the host segment as %3F reached the parser decoded,
so cdn.example.com?x=1 passed the allowlist as cdn.example.com, fetched
the host's root path with that query, and was cached under its own keys:
another unsigned spelling of one upstream request. The host segment is
now accepted only as a host name, an IPv4 address or a bracketed IPv6
address, with an optional port, and anything else is refused with 400.
An IPv6 address without brackets is now refused instead of needing a
signature; a host name outside ASCII is refused, its punycode accepted.

Model: opus-5-5

README.md

@@ -575,7 +575,11 @@ pixa lower-cases the host of a `/v1/image/` URL before anything else uses it, so
 `CDN.Example.com` is matched, fetched, cached and counted against
 `upstream_connections_per_host` as `cdn.example.com`. Such a URL whose host has
 a port, as in `cdn.example.com:8443`, or a `:` with no port, needs a signature
-even when the host is allowlisted. A host holding `@` is refused with 400.
+even when the host is allowlisted. The host must be a host name or an IPv4
+address (letters, digits, hyphens, underscores and dots) or an IPv6 address in
+brackets, either one optionally followed by `:` and a port. Any other host, such
+as one holding `@`, or a `?`, `#`, `%`, `\` or space written percent-encoded, is
+refused with 400.
 
 ### Configuration