Use SPDX license identifiers.
This patch switches code to SPDX one-line license identifiers according to https://spdx.dev/learn/handling-license-info/ and replacing long license text headers.
I used C++ format on the first line in style // SPDX-License-Identifier: except exported libcryptsetup.h, when only C comments are used.
The only additional changes are:
- switch backend utf8.c from LGPL2+ to LGPL2.1+ (as in systemd)
- add some additional formatting lines.
Next patch reorganizes license files.
This patch adds README.licensing describing used licenses. The license text files are now in dosc/licenses.
The main COPYING document in root is the default license, this also forces GitHub to properly identify default license.