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_agent_free.3

@@ -1,6 +1,5 @@
-.\"
 .\" Copyright (C) Daiki Ueno
-.\"
+.\" SPDX-License-Identifier: BSD-3-Clause
 .TH libssh2_agent_free 3 "28 May 2009" "libssh2" "libssh2"
 .SH NAME
 libssh2_agent_free - free an ssh-agent handle