npm / arborist / #210

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Log script executions and exits

There have been a few reports from users who were concerned and
perplexed that their install scripts didn't seem to be run, looking at
the logs npm produces.  In fact, the install scripts WERE running, just
running quietly and successfully.

This logs the commands Arborist runs explicitly, along with exit code
and signal, so that it's more clear what ran when we look at the logs.

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