support configurable response_mode, including "form_post"

added support for configurable response_mode ("fragment", "query" or
"form_post") server-wide defined (OIDCResponseMode) or per-client in
client metadata ("response_mode")

use "nonce" in all flows except for Google ("code" and "code token")
correct code comments in proto.c
check response mode on authorization response

check the requested response_mode against the one used in the response
and warn if they don't match; reorganized JWKS handling