[$] A ring buffer for epoll

The set of system calls known collectively as epoll wasdesigned to make polling for I/O events more scalable. To that end, itminimizes the amount of setup that must be done for each system call andreturns multiple events so that the number of calls can also be minimized.But that turns out to still not be scalable enough for some users. Theresponse to this problem, in the form of this patchseries from Roman Penyaev, takes a familiar form: add yet anotherring-buffer interface to the kernel.