dense node support
Update to latest autoconf-archive scripts.
Fix copy-and-paste typo in error message.
Removing GPLv3 source before merge as this project is 2-clause BSD.
Add visible flag for all dense nodes when writing history file.
Merge remote-tracking branch 'origin/master' into densenodes
Updated AUTHORS and ChangeLog.

Added Frederik to AUTHORS and added the news of dense nodes
support in PBF to the ChangeLog, which I've been forgetting to
maintain.
Better sanity checks and error messages in PBF writer.
Only check visible array on history files.
Use UID 0 for anonymous users in PBF dense nodes.
Fix up formatting.
Prefer throwing kind of assertion.
Explicit types on calls to delta, correct types for UIDs, const for checking, ensure last IDs always initialised.
Fixed a couple of warnings from -Wall.
Use own date formatting code to avoid std::locale overhead.
Use dense, specialised `int64_t` map to avoid overheads of `std::map`.
Merge branch 'master' into densenodes