Initial Async notify code changes

Initial API implemented for notifying applications that an ASYNC_JOB
has completed. Currently only s_server is using this. The Dummy Async
engine "cheats" in that it notifies that it has completed *before* it
pauses the job. A normal async engine would not do that.

Only the posix version of this has been implemented so far, so it will
probably fail to compile on Windows at the moment.

Reviewed-by: Rich Salz <rsalz@openssl.org>

ssl/ssl_lib.c

@@ -931,6 +931,14 @@ int SSL_waiting_for_async(SSL *s)
     return 0;
 }
 
+int SSL_get_async_wait_fd(SSL *s)
+{
+    if (!s->job)
+        return 0;
+
+    return ASYNC_get_wait_fd(s->job);
+}
+
 static int ssl_accept_intern(void *vargs)
 {
     struct ssl_async_args *args;