Handle overflow in initial window update in stream

Rename window_size in nghttp2_stream as remote_window_size.
Support increment/reduction of local window size by WINDOW_UPDATE
Allow disabling auto WINDOW_UPDATE for connection and stream individually

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 member of nghttp2_session as remote_window_size
Code cleanup
Update doc
Update doc
Remove priority adjustment for a DATA frame

Simply it does not work as expected.
Rewrite re-prioritization with PRIORITY
Disable local flow control in nghttp2_submit_window_update()
Early termination if stream is not found

Add this check in nghttp2_submit_{priority, window_update, data,
push_promise}.
src: Use print_frame_hd from print_data_frame
src: Print SETTINGS string name
src: Create strframetype to handle unknown frame types
src: Print opaque_data_len of GOAWAY
Update README.rst
Do not RST_STREAM for DATA against nonexistent stream

This may be useful to the misbehaving implementation, but it could
result in lots of RST_STREAM, so just ignore it for now.
Handle non-fatal error from nghttp2_submit_data
nghttpx: Save pid file after daemon() call
Update README.rst