intended to fix a problem reported by tb@: pkg-readmes docs used to have
the FULLPKGNAME encoded, which makes for gratuitous differences from version
to version.
- use PKGSTEM inside the README for less variation
- ditch the FULLPKGNAME as the install location, PKGSTEM is enough.
PKGSTEM ?= is overrideable, because stuff like php branches need to install
several readmes.
Next commit will fix all affected ports: all readmes change location, so lots
of plist tweaks and bumps
ee2d3810b4