callout_stop(), except that if the callout being stopped is currently
in progress, it blocks attempts to reset the callout and waits until the
callout is completed before it returns.
This makes it possible to clean up callout-using code safely, e.g.,
without potentially freeing memory which is still being used by a callout.
Reviewed by: mux, gallatin, rwatson, jhb
e0793884f3