install: Recursively set ownership of baseDir
internal: extract sl-pm-usage to usage file
install: add --set-env/-e for initial app env

Add the -e,--set-env option to sl-pm-install for setting initial env
vars for use by deployed applications. The environment specified by
this option is stored using the same internal store that the pmctl
get/set-env commands use.

The argument is parsed based on whitespace, including newlines, so it
is possible to do `slc pm-install --set-env "$(cat .env)"` to re-use
an existing .env file from a Heroku/12factor app.