Fix memory leak of SSL_CTX structures in TLS core.

SSL_CTX structures were allocated but never freed. This was a bigger
issue for clients than servers since new SSL_CTX structures could be
allocated for each connection. Servers, on the other hand, typically
set up a single SSL_CTX for their lifetime.

This is solved in two ways:

1. In __ssl_setup(), if a tcptls_cfg has an ssl_ctx on it, it is
freed so that a new one can take its place.
2. A companion to ast_ssl_setup() called ast_ssl_teardown() has
been added so that servers can properly free their SSL_CTXs.

(issue ASTERISK-19278)
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Merged revisions 367002 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 367003 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@367010 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/tcptls.h

@@ -196,8 +196,25 @@ void ast_tcptls_server_start(struct ast_tcptls_session_args *desc);
  * \version 1.6.1 changed desc parameter to be of ast_tcptls_session_args type
  */
 void ast_tcptls_server_stop(struct ast_tcptls_session_args *desc);
+
+/*!
+ * \brief Set up an SSL server
+ *
+ * \param cfg Configuration for the SSL server
+ * \retval 1 Success
+ * \retval 0 Failure
+ */
 int ast_ssl_setup(struct ast_tls_config *cfg);
 
+/*!
+ * \brief free resources used by an SSL server
+ *
+ * \note This only needs to be called if ast_ssl_setup() was
+ * directly called first.
+ * \param cfg Configuration for the SSL server
+ */
+void ast_ssl_teardown(struct ast_tls_config *cfg);
+
 /*!
  * \brief Used to parse conf files containing tls/ssl options.
  */