If all.do runs and creates no output, we shouldn't create a file called
'all', but we should remember that 'all' has been run successfully. We do
this by creating 'all.did' during the build.
Since minimal/do always just wipes everything out every time it runs, we can
safely remove the .did files after minimal/do terminates, so this doesn't
clutter things too much in normal use.
This fixes some edge cases, particularly that 'minimal/do clean' no longer
leaves stupid files named "clean" lying around, and the redo-sh directory
can now be rebuilt correctly since we rebuild it as long as redo-sh.did
doesn't exist. (We don't want to "rm -rf redo-sh" because it makes me
nervous.)
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