Add thread_run_flash() helper

It unsuspends single suspended thread, passed as the argument.
It is up to the caller to arrange the target thread to suspend later,
since the state of the process is not changed from stopped. In particular,
the unsuspended thread must not leave to userspace, since boundary code
is not prepared to this situation.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D29955