pending_local_max_concurrent_stream is, once local settings applied,
becomes NGHTTP2_INITIAL_MAX_CONCURRENT_STREAMS, which is very large
number.  When adjusting number of streams, we have to take min of
local effective SETTINGS_MAX_CONCURRENT_STREAMS and pending one.

Simplified when to send WINDOW_UPDATE to the backend, that is we send
WINDOW_UPDATE when input buffer is empty.

It appears that some Android client does not like Connection: close in
200 CONNECT response.  Browsers fine with this header field.  It is
better to remove it.  Squid does not emit it too.

Also update doc

It is generally useful to know what is the cause of the error.  Since
we expose HPACK API, it is friendly to tell application the
insufficient buffer size is a culprit.

Hide NGHTTP2_MAX_HEADER_TABLE_SIZE from public API.  Now it is defined
as ((1u << 31) - 1) in nghttp2_frame.h, which is sufficiently big
enough.

Merge branch 'add_check_compile_flag' of https://github.com/alagoutte/nghttp2 into alagoutte-add_check_compile_flag

Use this macro in Python sources.  Python module constant
HD_DEFLATE_HD_TABLE_BUFSIZE_MAX was renamed as
DEFLATE_MAX_HEADER_TABLE_SIZE since the previous name was awkward.

* -Wpragmas
* -Wunreachable-code
* -Wpedantic
* -Waddress
* -Wattributes
* -Wdiv-by-zero
* -Wheader-guard

It's fix the build with Clang and --enable-werror, don't support -WClobberred and display error about unknown warning option
error: unknown warning option '-Wclobbered'; did you mean '-Wconsumed'? [-Werror,-Wunknown-warning-option]