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