Mention client-ciphers, and no-http2-cipher-black-list options in
psk-secrets and client-psk-secrets options.

Previously, ciphers option sets cipher list for both frontend and
backend TLS connections.  With this commit, ciphers option only sets
cipher list for frontend connections.  The new client-ciphers option
sets cipher list for backend connection.

This commit adds client-no-http2-cipher-black-list option to disable
enforcement of HTTP/2 cipher black list on backend HTTP/2 connection.
Previously, existing no-http2-cipher-black-list option disables it for
both frontend and backend connections.  Now no-http2-cipher-black-list
option only disables it for frontend connection.

Because of the redundant check in backend HTTP/2 session,
no-http2-cipher-black-list does not work on backend HTTP/2 connection.
This commit fixes it.

Fix issue reporting by PVS Studio

Update README file

Consider checking the fourth actual argument of the 'fprintf' function. The SIGNED integer type argument is expected.

nghttp2_hd: fix It is odd that the body of 'hd_get_num_table_entries' function is fully equivalent to the body of 'get_max_index' function found by PVS Studio (V524)

nghttp2_submit: fix Parameter 'flags' is always rewritten in function body before being used found by PVS Studio (V763)

Previously, global accept-proxy-protocol option enables PROXY protocol
support for all frontend listeners, but this was inflexible.  To fix
this issue, accept-proxy-protocol option is now deprecated, and
instead proxyproto parameter in frontend option enables PROXY protocol
support per frontend.

boringssl says:

/* It is an error to clear any bits that have already been set. (We can't try
 * to get a second close_notify or send two.) */
assert((SSL_get_shutdown(ssl) & mode) == SSL_get_shutdown(ssl));

Plain write(2) is OK, but SSL_write requires same arguments on retry.
It would be better to avoid calling them again.

nghttpx: Add frontend-keep-alive-timeout option