Write access improvements

Implemented write access settings BLOCKED, ADMIN and TOKEN.

SQL/MED functions for getting URL parts now work on text as well as datalinks. For example, you can use dlurlcomplete(url) to normalize URLs.

You can now also convert relative to absolute links with dlvalue(url,dlvalue(base)).

Datalinker daemon is improved, with less bugs.

Some renaming and internal reorganization. datalink.column_options is now datalink.columns.