We use PYTHON variable to execute Python script, mainly to generate
documents.  Therefore it is better to always check python interpreter
even if we don't request python bindings

The == operator is not in POSIX; use = like normal.
Reported-by: Mikael Magnusson <mikachu@gmail.com>

Client certificates can use used for authentication/authorization in the server,
so expose them similar to the remote address.

The private global variable nghttp2_enable_strict_preface is also
marked as NGHTTP2_EXTERN, but it is test purpose only (test with
.dll), and not part of public API.  It could be removed in the future
release.

This commit makes sure that WebSocket upgrade works for HTTP/1.1
frontend and backend pair.  Actually, this implementation probably
supports other upgrade as well, other than HTTP/2 Upgrade, which is
handled specially in other place.

Setting TCP_NODELAY may fail. Instead of failing the routine, log it and continue.
It seems, that setting TCP_NODELAY in python on Mac OS X is not necessarily supported.