Separate extension frames from core frames

ALTSVC and BLOCKED frames are now extension frames.  To add new
extension frame without modifying nghttp2_frame union, which causes so
name bump, we separated extension frames from core frames.
nghttp2_frame includes generic nghttp2_extension.  The payload member
of nghttp2_extension will point to the structure of extension frame
payload.  The frame types of extension frames are defined in
nghttp2_ext_frame_type.
Fix compile error with --enable-debug
Fix unused header_cb_arg structure