Function dt_proc_waitrun

* Wait for a stopped process to be set running again by some other debugger. * This is typically not required by /proc-based debuggers, since the usual * model is that one debugger controls one victim. But DTrace, as usual, has * its own needs: the stop() action assumes that prun(1) or some other tool * will be applied to resume the victim process. This could be solved by * adding a PCWRUN