[sandbox-null.c sandbox-rlimit.c sandbox-seccomp-filter.c]
[sandbox-systrace.c ssh-sandbox.h sshd.c] Support preauth sandboxing
using the Capsicum API introduced in FreeBSD 10. Patch by Dag-Erling
Smorgrav, updated by Loganaden Velvindron @ AfriNIC; ok dtucker@