Add support for loading single AT patterns in AccessMap

In addition to filenames and URLs, loadAccessTransformer(String) now
accepts the literal "pattern:" followed by an access transformer line.
Useful for adding individual transformations without a new _at.cfg.
Add convenient alias to private visibility

Since AccessMap never downgrades visibility, 'private' in AT is
effectively equivalent to 'no change'. Useful in case you want to
change other flags but not the visibility. Accept '*' as private.
Add support for access map '**' pattern to match all symbols

Classes, fields, and methods. Also add some documentation of
this ever-expanding file format.