Add support for TLS Next Protocol Negotiation:

* modules/ssl/mod_ssl.c, modules/ssl/mod_ssl.h: Add and implement new
  hooks for next protocol advertisement/discovery.

* modules/ssl/ssl_engine_init.c (ssl_init_ctx_callbacks): Enable
  NPN advertisement callback in handshake.

* modules/ssl/ssl_engine_io.c (ssl_io_filter_input): Invoke
  next-protocol discovery hook.

* modules/ssl/ssl_engine_kernel.c (ssl_callback_AdvertiseNextProtos): 
  New callback.

* modules/ssl/ssl_private.h: Add prototype.

Submitted by: Matthew Steele <mdsteele google.com>
  with slight tweaks by jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1332643 13f79535-47bb-0310-9956-ffa450edef68