[Backtracing] Harden output path setting.

We should harden the output path setting so that it can't be used
as part of an exploit to get a crashing process to overwrite a
file at an attacker-controller path, or to divert the crash report
to `/dev/null` to hide their tracks or other such undesirable
activity.

Take a copy of the setting at start-up and write-protect it to
prevent attackers overwriting it.

Note that we already protect against attempts to trigger the
backtracer from privileged programs (both on Darwin and Linux);
this is really a belt and braces measure to make life harder for
attackers.

rdar://136977833

Author: al45tair