Improve handling of SIGINT.

[reitermarkus]

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1. Make ignore_interrupts raise an Interrupt exception afterwards if there was an an attempt to interrupt it.

   If I press CTRL-C, I expect brew to exit after the “One sec, cleaning up...” message and not continue running until I happen to find a “correct” position where pressing CTRL-C actually works.

2. Change SystemCommand to spawn processes in another process group, so that the parent process can send SIGINT to the child rather than the child receiving it directly. We want to raise an Interrupt exception rather than the child exiting with a Process::Status containing only a signal but no actual exit status.

[BrewTestBot]

Review period will end on 2020-12-18 at 16:11:03 UTC.

[BrewTestBot]

Review period ended.