ProsperoStore (PPSA99000) imports kill from libkernel and stops at it on
current main. Its crash reporter resets a crash signal to SIG_DFL and
sends it again with kill(getpid(), signal).
kill follows FreeBSD's kern_kill and killpg1 for the only guest process:
EINVAL for signals outside 0 to 128, signal 0 only checks the target,
and the caller's own pid, 0 and minus its pid deliver through raise.
Any other pid, and -1, which skips the caller, report ESRCH.
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