Following cases are now treated as connection error of type
PROTOCOL_ERROR.

* Receiving HEADERS from client in reserved (local)
* Receiving PUSH_PROMISE against peer-initiated stream
* Receiving WINDOW_UPDATE against in reserved (local, remote)

If multiple same ID are found, use the last one.

If multiple required headers (e.g., :path) found, return HTTP 400
error.
Fix util::strieq(a,b,n) where boundary of b is not checked in the
loop.

This may be useful to the misbehaving implementation, but it could
result in lots of RST_STREAM, so just ignore it for now.

Add this check in nghttp2_submit_{priority, window_update, data,
push_promise}.