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>

checksum.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>
+ */
+
 uint32_t sum_16b(void *buf, size_t len);
 uint16_t csum_fold(uint32_t sum);
 uint16_t csum_unaligned(void *buf, size_t len, uint32_t init);