Downstream's headers mutation functions have been rewritten to share
code.

nghttp2_submit_request and nghttp2_submit_response will set
NGHTTP2_FLAG_END_STREAM after all given data is sent (data could be
0).  This means we have no way to send trailers.  In this commit, we
added NGHTTP2_DATA_FLAG_NO_END_STREAM flag.  The application can set
this flag in *data_flags inside nghttp2_data_source_read_callback.  If
NGHTTP2_DATA_FLAG_EOF is set, library automatically set
NGHTTP2_FLAG_END_STREAM.  But if both NGHTTP2_DATA_FLAG_EOF and
NGHTTP2_DATA_FLAG_NO_END_STREAM are set, NGHTTP2_FLAG_END_STREAM will
not set by library.  Then application can use new
nghttp2_submit_trailer() to send trailers.  nghttp2_submit_trailer()
will set NGHTTP2_FLAG_END_STREAM and it is actually thing wrapper of
nghttp2_submit_headers().

After we sent SETTINGS including ENABLE_PUSH = 0, peer may already
issue PUSH_PROMISE before receiving our SETTINGS and react it to
SETTINGS ACK.  Previously we accept this PUSH_PROMISE.  In this
commit, we check the pending ENABLE_PUSH value and if it means
disabling push, we refuse PUSH_PROMISE with RST_STREAM of error
REFUSED_STREAM.