The compiler may advertise function attribute support
with __has_attribute operator even though it does
not implement the feature on some architecture.
This fixes the issue with GCC 11 on ppc64le with
__attribute__((zero_call_used_regs("used"))).
Fixes: #959.
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: <id>
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.
