Merge r1890605, r1893164, r1893179 from trunk:

Merge r1890605, r1893164, r1893179 from trunk:

  *) core/mod_proxy/mod_ssl:
     Adding `outgoing` flag to conn_rec, indicating a connection is
     initiated by the server to somewhere, in contrast to incoming
     connections from clients.
     Adding 'ap_ssl_bind_outgoing()` function that marks a connection
     as outgoing and is used by mod_proxy instead of the previous
     optional function `ssl_engine_set`. This enables other SSL
     module to secure proxy connections.
     The optional functions `ssl_engine_set`, `ssl_engine_disable` and
     `ssl_proxy_enable` are now provided by the core to have backward
     compatibility with non-httpd modules that might use them. mod_ssl
     itself no longer registers these functions, but keeps them in its
     header for backward compatibility.
     The core provided optional function wrap any registered function
     like it was done for `ssl_is_ssl`.

  *core: clarify comments and use hook API better to check for presence of callbacks.

 * optimizing hook check as suggested by Yann.

Submitted by: icing
Reviewed by:

Github: closes #203

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1893182 13f79535-47bb-0310-9956-ffa450edef68