- expose MISSING_FILES
- treat them as a special kind of IGNORE
- handle that in DPB
that way, ports that are IGNORE'd for other reasons (not correct arch)
stay ignored, and relevant ports with missing distfiles get better error
messages
- clean up clock handling, jobs with timer.
- pass more parameters thru state
- tweak the engine to handle distfiles
- lots of new code in Fetch for new jobs.