- we now only create them when building X11, and only create the ones
we need (X11R6 xor X11R7)
- all these subdirs are now in the xbase set
- move the logic for running mtree into etc/mtree/Makefile
- split NetBSD.dist into 3 files, and have the build and postinstall handle
creating a possibly merged one. we still have a single installed file
called "NetBSD.dist".