Instead use (void)VAR for better compiler compatibility.

This commit fixes the bug in nghttp2_session_want_write.  Previously,
it may return 0 if there is pending frames after GOAWAY frame is
submitted.

To avoid the situation that nghttp2_session_want_write keeps returning
nonzero after GOAWAY and the number of active streams is 0 (e.g., keep
receiving SETTINGS or PING), nghttp2_session_mem_recv now just
swallows the input data without parsing in this case.

nghttpx: Enable backend pattern matching with http2-proxy

.. after fork but before execv in multithreaded process.

nghtt2_gzip: fix this statement may fall through [-Werror=implicit-fa…

Previously, nghttpx will use only one single thread inside the worker
process if --workers=1 (this is default).  If --workers=N, N > 1, we
use additional threads for accepting connections, or API request
processing, etc.

With this commit, we use the same processing model for N > 1 even if N
== 1.  To restore the original single thread execution mode,
--single-worker option is added.  If threading is disabled
--single-worker is always true.