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]

We simulate resource sharing by decreasing weight.  The thing is if
weight is wrapped, that item continues to send DATA until its weight
gets lowered under the other items.  This commits fix this issue.

Since we have stream ID immediately available from nghttp2_submit_*,
we don't need adjust_priority_callback.

Previously stream ID was assigned just before HEADERS or PUSH_PROMISE
was serialized and nghttp2_submit_{request, headers, push_promise} did
not return stream ID.  The application has to check assigned stream ID
using before_frame_send_callback.  Now it is apparent that priority is
meant to DATA transfer only.  Also application can reorder the
requests if it wants. Therefore we can assign stream ID in
nghttp2_submit_* functions and return stream ID from them.  With this
change, now application does not have to check stream ID using
before_frame_send_callback and its code will be simplified.

timegm.h:30:8: error: C++ style comments are not allowed in ISO C90 [-Werror]