Cleaner method for reinvoking a running script

[OSXLich-Doug]

when_reinvoked {|proc| ... }

If set, a future attempt at invoking this script while it is already running calls proc instead (rather than the ;force soandso stuff). It'd be cleaner than the current ways of getting around that (mostly upstream hooks), particularly since it'd work with alias/do_client/etc

All praise the LR