ports/git: Improve rules for git-subtree

Follow the directions in contrib/subtree/INSTALL. And run `make test`
too.
waffle: Use platform autodetection

waffle master can now autodetect which platforms are available at
configure time.
ports/llvm: Disable subprojects that don't play well with Linux
README: Warn of instability
kport: Use GNU stow to manage files

The miserably buggy kiwiports database is made redundant by stow.