Add license type UnspecifiedLicense.

Before this, AllrightsReservered had two separate meanings; the author
explicitly chose this license or not license was specified and therefore
defaults to AllRightsReserved.

The default license when no license is specified is now UnspecifiedLicense.

Closes: #2141
Shouldn't this be `PackageDistSuspicious`? I think we don't want to ban `AllRightsReserved` outright.