Plug memory disclosures via ptrace(2).

On some architectures, the structures returned by PT_GET*REGS were not
fully populated and could contain uninitialized stack memory. The same
issue existed with the register files in procfs.

Reported by: Thomas Barabosch, Fraunhofer FKIE
Reviewed by: kib
MFC after: 3 days
Security: kernel stack memory disclosure
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18421