Add Public Domain support to ports license framework. You can now use

LICENSE=  PD

Note that although Public Domain is not technically a license, it's
handled in the same way as licenses here, which is a common practice
(Arch, Gentoo, Fedora, Debian, even FOSSology do the same).

Convert all ports which redefine Public Domain LICENSE to LICENSE=PD.

Approved by: portmgr (bapt)
Differential Revision: D4149