MINOR: ssl_sock: implement and use prepare_srv()/destroy_srv()

Now we can simply check the transport layer at run time and decide
whether or not to initialize or destroy these entries. This removes
other ifdefs and includes from cfgparse.c, haproxy.c and hlua.c.
diff --git a/src/haproxy.c b/src/haproxy.c
index adffda9..611371c 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -1446,10 +1446,11 @@
 			free(s->agent.bo);
 			free(s->agent.send_string);
 			free((char*)s->conf.file);
-#ifdef USE_OPENSSL
-			if (s->use_ssl || s->check.use_ssl)
-				ssl_sock_free_srv_ctx(s);
-#endif
+
+			if (s->use_ssl || s->check.use_ssl) {
+				if (xprt_get(XPRT_SSL) && xprt_get(XPRT_SSL)->destroy_srv)
+					xprt_get(XPRT_SSL)->destroy_srv(s);
+			}
 			free(s);
 			s = s_next;
 		}/* end while(s) */