Stop requiring use of outdated/confusing term 'legacy' in access.php

See attached patch. This makes it possible to use the word 'archetypes' for the list of archetypes that should have a capability by default, instead of 'legacy'.

You can still use 'legacy' as well so there is no problem for backward capability.

I tested by creating a new capability using this name, seems to work ok.

If this patch is approved I am happy to search/replace all uses of 'legacy' in access.php with 'archetypes' in core code as well, in order to encourage this as a new coding style.

Just seems to me pretty awful that we use the word 'legacy' in those files, and moving to 2.0 is a nice point to change the coding style (while keeping it so the old code still works).