Set TDP_KTHREAD before calling cpu_fork() and cpu_copy_thread().

This permits these routines to use special logic for initializing MD
kthread state.

For the kproc case, this required moving the logic to set these flags
from kproc_create() into do_fork().

Reviewed by: kib
MFC after: 1 week
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D29207