rc.d and motd warnings if WITNESS, INVARIANTS, malloc debugging, etc., are enabled

[rwatson]

With a relatively new community of CheriBSD users forming, who are generally expected to be inexperienced with FreeBSD, we need to make it extremely clear when performance-harmful debug options are enabled by default in the kernel and userspace. Two possibilities, likely both of which should be pursued, are:

• An rc.d script running at the very end of the boot that checks various relevant sysctls and libc options
• An rc.d script that updates /etc/motd to include debugging feature warnings at login when they are enabled