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

cmake/CheckNonblockingSocketSupport.cmake

@@ -1,3 +1,5 @@
+# Copyright (C) The libssh2 project and its contributors.
+# SPDX-License-Identifier: BSD-3-Clause
 include(CheckCSourceCompiles)
 
 # - check_nonblocking_socket_support()