This saves a system call and avoids one of the (relatively rare) cases
of the kernel exporting pointers via sysctl.
As a temporary measure, keep the sysctl support to allow limited
compatability with old kernels.
Fail gracefully if ps_strings can't be found (should never happen).
Reviewed by: kib
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24407
3076591b7a