Add support for ps -H on corefiles in libkvm

Add support for kernel threads in kvm_getprocs() and the underlying
kvm_proclist() in libkvm when fetching from a kernel core file. This
has been missing/needed for several releases, when kernel threads became
normal threads. The loop over the processes now contains a sub-loop for
threads, which iterates beyond the first thread only when threads are
requested. Also set some fields such as tid that were previously
uninitialized.

Reviewed by: vangyzen jhb(earlier revision)
MFC after: 4 days
Sponsored by: Forcepoint LLC
Differential Revision: https://reviews.freebsd.org/D21461