Move mod_ssl-internal interfaces into ssl_private.h; allow mod_ssl.h

Move mod_ssl-internal interfaces into ssl_private.h; allow mod_ssl.h
to be included even when mod_ssl is not enabled.

* Makefile.in (install-include): Only install mod_ssl.h.

* modules/ssl/ssl_private.h: New file.

* modules/ssl/mod_ssl.h: Move everything apart from than the optional
hook definitions into ssl_private.h.

* modules/ssl/*.c: Include ssl_private.h not mod_ssl.h

* modules/ssl/config.m4: Always add the mod_ssl directory to the
include path so other modules can find mod_ssl.h.

* modules/proxy/mod_proxy.c: Include mod_ssl.h to pick up the optional
hook definitions rather than copy'n'pasting them.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@102803 13f79535-47bb-0310-9956-ffa450edef68