hpackcheck.py: Remove sorting to check ordering requirements
Add some header names which won't be indexed
nghtp2_hd: Calculate hash values once
src: Remove http2::sort_nva

This function is no longer necessary because 0x00 concatenation rule
is gone.
nghttp: Check HTTP header field characters
nghttpx, nghttpd: Check pseudo header fields come before normal header fields
src: Sort header fields so that pseudo headers come first
nghttp: Sort request header fields using http2::name_less
Add Wdocumentation Clang Flag