http, mod_ssl: Introduce and return the 421 (Misdirected Request) status code

http, mod_ssl: Introduce and return the 421 (Misdirected Request) status code
for clients requesting a hostname on a reused connection whose SNI (from the
TLS handshake) does not match.
PR 5802.

This allows HTTP/2 clients to fall back to a new connection as per:
https://tools.ietf.org/html/rfc7540#section-9.1.2

Proposed by: Stefan Eissing <stefan eissing.org>
Reviewed by: ylavic

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1685069 13f79535-47bb-0310-9956-ffa450edef68