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}.

Simply it does not work as expected.

Now NGHTTP2_OPT_NO_AUTO_WINDOW_UPDATE is split into 2 options:
NGHTTP2_OPT_NO_AUTO_STREAM_WINDOW_UPDATE and
NGHTTP2_OPT_NO_AUTO_CONNECTION_WINDOW_UPDATE.
This is preparation for the upcoming removal of END_FLOW_CONTROL
flag. For nghttpx, instead of using END_FLOW_CONTROL to disable
connection-level flow control, increase window size by large
enough value, which is friendly way to current chromium
implementation.

Rename window_size in nghttp2_stream as remote_window_size.

Fix the bug that connection-level local window is not updated
for the data is the last part of the stream. For the stream
level window may ignore this, connection-level window must
be updated. Also this change fixes the bug that connection-level
window is not updated for the ignored DATA frames.

The max length of header block is not used right now. It will
be used when header continuation is implemented.