Update r332860 by changing the default from suppressing post-panic
assertions to not suppressing post-panic assertions.

There are some post-panic assertions that are valuable and we shouldn't
default to disabling them. However, when a user trips over them, the
user can still adjust the tunable/sysctl to suppress them temporarily to
get conduct troubleshooting (e.g. get a core dump).

Reported by: cem, markj