Fix typo

This is regression introduced in
46b70c1d.

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.