nghttpx: Preserve upgrade in Connection header field
Add API for HTTP Upgrade
Change NGHTTP2_INITIAL_MAX_CONCURRENT_STREAMS

The previous value 100 is non-standard, and if it is not sent with
SETTINGS, session will be closed with protocol error.
nghttp, nghttpx: Add HTTP Upgrade from HTTP/1.1 to HTTP/2.0

nghttpx does not perform upgrade if the request has request body.
nghttpx: Rename --{front,back}end-spdy-no-tls as --{front,back}end-no-tls
nghttpx: Rename spdy_{upstream,downstream}_no_tls as {upstream,downstream}_no_tls
nghttpx: Code cleanup
nghttp: Ignore -u if --no-tls is not given
nghttpx: Update help messages
Update README.rst
src: Make window size - 1

To match the -w16 to 65535, which is HTTP/2.0 default initial window
size, decrement 1 from (1 << window_bits).