netops: on SSL teardown only send shutdown alert

According to man 3 SSL_shutdown / TLS, "If a unidirectional shutdown is
enough (the underlying connection shall be closed anyway), this first
call to SSL_shutdown() is sufficient."

Currently, an unidirectional shutdown is enough, since
gitno_ssl_teardown is called by gitno_close only. Do so to avoid further
errors (by misbehaving peers for example).

Fixes #1129.

src/netops.c

@@ -198,10 +198,7 @@ static int gitno_ssl_teardown(gitno_ssl *ssl)
 {
 	int ret;
 
-	do {
 	ret = SSL_shutdown(ssl->ssl);
-	} while (ret == 0);
-
 	if (ret < 0)
 		ret = ssl_set_error(ssl, ret);
 	else