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

example/ssh2.c

@@ -11,6 +11,8 @@
  *  -i authenticate using keyboard-interactive
  *  -k authenticate using public key (password argument decrypts keyfile)
  *  command executes on the remote machine
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
  */
 
 #include "libssh2_setup.h"