Files which are installed as-is (any .service and other unit files, .conf
files, .policy files, etc), are left as is. My assumption is that SPDX
identifiers are not yet that well known, so it's better to retain the
extended header to avoid any doubt.
I also kept any copyright lines. We can probably remove them, but it'd nice to
obtain explicit acks from all involved authors before doing that.
This is essentially a revert of f38e0cce75 (which
removed the documentation of DUIDType on purpose). The description is heavily
updated for the new semantics.
This addresses #3127 § 4.
1. Replace strtol with unhexchar, verified with valid and invalid DUID strings.
2. Fix logging to use log_syntax instead of log_error.
3. On error reading DUID, ignore read and preserve previous state.
4. Fix man-pages to use markup, remove options not yet implemented.
5. Remove spurious header line in new files.
