Mark procfs-based process debugging as deprecated for FreeBSD 13.

Attempting to use ioctls on /proc/<pid>/mem to control a process will
trigger warnings on the console. The <sys/pioctl.h> include file will
also now emit a compile-time warning when used from userland.

Reviewed by: emaste
MFC after: 1 week
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D23822