Attempt to use AT_PS_STRINGS to get the ps_strings pointer.

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