system(): Don't use /dev/klog in a FreeBSD jail

When figuring out what to expand system() to, check whether we're in a
jail when running on FreeBSD, and don't include /dev/klog if we are.
This fixes #93.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #98 from algernon/f/system/freebsd-jail

system(): Don't use /dev/klog in a FreeBSD jail