Take advantage of `resume` restart added in R3.4

[brodieG]

Hopefully this allows us to get around some of the ungraceful exits we're subject to currently. From NEWS:

In most cases user interrupt handlers will be called with a "resume" restart available. Handlers can invoke this restart to resume computation. At the browser prompt the r command will invoke a "resume" restart if one is available. Some read operations cannot be resumed properly when interrupted and do not provide a "resume" restart.