Teach `bin/ci.sh` to cache the git commit hash of the last successful
build.  Use the cached hash to determine whether building is even
necessary.  This feature introduces a new variable, `HBSD_CACHEDIR`, to
use as cache storage for the build.

Introduce `bin/cleanup.sh`. This script isn't even close to being
complete.

Signed-off-by: Shawn Webb <shawn.webb@hardenedbsd.org>