With gccgo some operating systems require using select rather
than epoll or kevent.  Using select means that we have to wake
up the polling thread each time we add a new file descriptor.
This implements that in the generic code rather than adding
another wakeup channel, even though nothing in the current net
package uses the capability.

R=rsc, iant2
CC=golang-dev
https://golang.org/cl/4284069