[aur-general] license convention for public domain packages

There are widely varying methods for specifying the license of a public domain package in Arch Linux. We should standardise and use one of them. Some packages use - 'Public Domain' (unclutter, python-webpy) - 'PD' (ttf-mph-2b-damase) - I think some packages might also be using 'none'. I saw one package using 'custom:public' (shuffle) Also, there is the question of whether we should have public domain declarations for each package in /usr/share/licenses or put a public domain declaration in /usr/share/licenses/common and refer to that. -- Abhishek