Somewhat inefficient (deep check of package vs port signature).
More proper approach would be to tell the engine to always build, have first
step be a check for signature if the package exist, and abort if it matches:
- here we double check everything
- propagating rebuilds means non-uptodate packages give rise to non uptodate.
- if several cores are available, recheck errors, so that we're not stuck
waiting for a core to finish to start new shit.
- $logdir/stop-<machine_name> allows you to stop a given machine while
you clean it up and (possibly) restart it (e.g., won't start new jobs)
turns out we probably don't need FULLSUBDIR after all.
compute SUBDIR better, possibly with quotes.
Remove trailing / on subdirs, in case people use completion.
