provide SPDX identifiers

- All files have prominent copyright and SPDX identifier
- If not embedded in the file, in the .reuse/dep5 file
- All used licenses are in LICENSES/ (not shipped in tarballs)
- A new REUSE CI job verify that all files are OK

Assisted-by: Viktor Szakats

Closes #1084

docs/libssh2_base64_decode.3

@@ -1,3 +1,5 @@
+.\" Copyright (C) The libssh2 project and its contributors.
+.\" SPDX-License-Identifier: BSD-3-Clause
 .TH libssh2_base64_decode 3 "23 Dec 2008" "libssh2 1.0" "libssh2"
 .SH NAME
 libssh2_base64_decode - decode a base64 encoded string