This commit fixes the bug that x-forwarded-proto header field sent on
HTTP/2 backend always "http", regardless of frontend scheme.

Unless method is CONNECT, we require that :path starts with "/",
except for OPTIONS method, which can take "*" as :path (server-wide
OPTIONS request).

When we know that stream is closed at time we read DATA frame header,
we use NGHTTP2_IB_IGN_DATA, and consume data for connection if
nghttp2_option_set_no_auto_window_update() is used.  However, if
stream is closed while we are in NGHTTP2_IB_READ_DATA, those bytes are
not consumed for connection, nor notified to application via callback,
so it eventually fills up connection window and connection will
freeze.  This commit fixes this issue by consuming these data for
connection when stream is closed or does not exist.

We share the same method value with http-parser.  This commit also
returns 501 for unknown request method on HTTP/2 and SPDY frontend.

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>