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The watcher should also watch for signals like SIGTERM killing sidekiq, which
are trapped by sidekiq, with sidekiq exiting successfully (with exit code 0).

To achieve this we rework our watcher-sigkill to be a generic watcher -
that can be given a set of restart exit codes including signal names and
monitors whether child process terminated with matching for restart exit
code.

Example usage:

	watcher 0,SIGKILL prog ...

Based on patch by @iv.
Discussion: https://lab.nexedi.com/lab.nexedi.com/lab.nexedi.com/issues/25#note_22085