LICENSES: Add license text files, add missing notices, fix SPDX tags

SPDX tags don't replace license files. Some notices were missing and
some tags were not according to the SPDX specification, too.

Now reuse --lint from the REUSE tool (https://reuse.software/) passes.

Reported-by: Martin Hauke <mardnh@gmx.de>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

pasta.h

@@ -1,3 +1,8 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later
+ * Copyright (c) 2021 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ */
+
 void pasta_start_ns(struct ctx *c);
 void pasta_ns_conf(struct ctx *c);
 void pasta_child_handler(int signal);