Add per-process flag to disable logsigexit

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.

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."

Reviewed by: kib (earlier version), pstef
Differential Revision: https://reviews.freebsd.org/D21903