Improve port API and update generic_unix socket_driver to use it

[pguyot]

Add port_ensure_available_with_roots after memory_ensure_free_with_roots Add several new port_send_* functions Update port_send_reply to ensure memory is available Document port_send_* functions Use these new functions in generic_unix's socket_driver

These changes are made under both the "Apache 2.0" and the "GNU Lesser General Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later