[$] SFrame-based stack unwinding for the kernel

The kernel's perfevents subsystem can produce high-quality profiles, with fullfunction-call chains, of resource usagewithin the kernel itself. Developers, however, often would like to seeprofiles of the whole system in one integrated report with, for example,call-stack information that crosses the boundary between the kernel anduser space. Support for unwinding user-space call stacks in the perfevents subsystem is currently inefficient at best. A long-running effortto provide reliable, user-space call-stack unwinding within the kernel,which will improve that situation considerably, appears to be reachingfruition.