Remove NPN support and focus on ALPN (RFC 7301)

Remove NPN support and focus on ALPN (RFC 7301)

* modules/ssl/mod_ssl.c, modules/ssl/mod_ssl.h: drop
  modssl_register_npn optional function and related declarations.

* modules/ssl/ssl_engine_init.c (ssl_init_ctx_callbacks):
  no longer set NPN advertisement callback.

* modules/ssl/ssl_engine_io.c (ssl_io_filter_input): remove
  NPN handling.

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

* modules/ssl/ssl_private.h: remove NPN prototypes, set
  HAVE_TLS_ALPN (OpenSSL 1.0.2 and later) with feature-based detection.

Rename SSLAlpnPreference to SSLALPNPreference, and add documentation.

Previous commits related to NPN and ALPN, for reference purposes:

r1332643 - Add support for TLS Next Protocol Negotiation
r1487772 - mod_ssl: Redesign NPN (Next Protocol Negotiation) API
           to avoid use of hooks API and inter-module hard linkage
r1670397 - ALPN support, based on mod_spdy/mod_h2 patch set
r1670434 - More ALPN goodness

(plus some minor tweaks: r1670578, r1670440, r1670578,
 r1670738, r1675459, and r1675549)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1676004 13f79535-47bb-0310-9956-ffa450edef68