Move header name/value pair validation to on_*_received functions
src: Omit \r\n sanitization on header value

This is because nghttp2 and spdylay now don't allow \r\n in header
value. Require spdylay >= 1.2.3.
configure.ac: Print out spdylay availability
Rename nghttp2_session_fail_session as nghttp2_session_terminate_session
Update doc