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.