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>

netlink.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>
+ */
+
 int nl_sock_init(struct ctx *c);
 unsigned int nl_get_ext_if(int *v4, int *v6);
 void nl_route(int ns, unsigned int ifi, sa_family_t af, void *gw);