I added a third value for kern.logsigexit to mean 'auto' as an abundance of caution, but I don't know how much it matters -- that can be easily consolidated back to boolean-ish. I'm also unsure of who else to add to this review, other than kib.
This is primarily targeted towards people running test suites under CI (e.g. buildbot, jenkins). Oftentimes tests entail segfaults that are expected, and logs get spammed -- this can be particularly high volume depending on the application. Per-process control of this behavior is desirable because they may still want to be logging legitimate segfaults, so the system-wide atomic bomb kern.logsigexit=0 is not a great option.
This adds a process flag to disable it, controllable via procctl(2)/proccontrol(1); the latter knows it as "sigexitlog" due to its length, but it's referred to almost everywhere else as "sigexit_log."