simplify job handling a great deal: we don't care when jobs get

stopped/continued, as we won't try to start new jobs when other stuff
is stopped.

Redo signal handling so that most stuff can be done directly in the handler.
This requires blocking/unblocking signals while creating new jobs, and creating
a small list that only contains job's pids.

Switch to pgrps for jobs, since that works.

Add a clamping heuristic that avoids starting new jobs while an expensive job
is running (expensive meaning "very likely to be a recursive make run"). This
idea is mostly from Theo, through the implementation is mine.

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