Correct $OpenBSD$ stuff

changes to get target equivalence to work better.
- add new file to create lists of equivalent targets (siblings)
- use that for sequential mode to have much better VPATH support
- separate checking

changes to get target equivalence to work better.
- add new file to create lists of equivalent targets (siblings)
- use that for sequential mode to have much better VPATH support
- separate checking commands from reporting error, for later.
- zap DieHorribly accordingly
- renumber existing flags
- signal_running_jobs() is simpler than pass_signal_to_jobs()
- new debug option -dn for name matching.

Similar code to handle parallel make is still missing.

thanks to Mark, Miod, Theo, Otto, Todd for tests and/or comments.

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