Get build and restore options from environment; provide set-quiet target
- see #123
- leaves default build unchanged
- run `build set-quiet default` for a much-quieter build
Address PR comments
- "K_build_options" -> "KPM_build_options" environment variable
- "K_build_options" -> "KPM_restore_options" (oops)
- "set-quiet" -> "--quiet" target

nit: remove trailing whitespace