If thread requested to not stop on non-boundary, then not only
stopping signals should obey, but also all forms of single-threading.
Otherwise, thread might sleep interruptible while owning some
resources, and single-threading thread could try to access them.
An example is owning vnode lock while dumping core.

Submitted by: Conrad Meyer
Review: https://reviews.freebsd.org/D2612
Tested by: pho
MFC after: 1 week