Previously, we did not handle PRIORITY frame which depends on itself
and for idle stream.  As a result, nghttp2_session_mem_recv (or
nghttp2_session_recv) returne NGHTTP2_ERR_NOMEM.  The error code was
still misleading.  It was not out of memory, and we failed to insert
hash map because of duplicated key, which was treated as out of
memory.  This commit fixes this issue, by explicitly checking
dependency for incoming PRIORITY for all cases.

While this commit optimizes dependency routine to Firefox style tree,
the other use cases (e.g., linear chain) are also improved
dramatically as well.

Typo

This commit adds special handling of IPv6 literal with zone ID as per
RFC 6874.  Still IPv6 link local address does not work, since URI
parser from http-parser does not allow this syntax.

Since this library uses the third-party subdir, make sure we auto-enable
it when that lib is turned on.

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