Extract authenticate/probe-resistance to auth.go and roundTrip/forwarding
to proxy.go, following the handler/http/ pattern. Export sentinel errors
(ErrAuthFailed, ErrWrongConnType) for precise test assertions. Fix
flushWriter panic recovery to handle non-string/non-error panics. Default
HTTPS port to 443 when no port is specified in the Host header.
Knock previously accepted a single hostname. Now it accepts multiple
comma-separated hostnames; probe resistance is bypassed if the request
hostname matches any entry in the list. Matching is case-insensitive
and whitespace around entries is trimmed.
ginuerzh