net: permit pollster DelFD to return whether to call Wakeup

This is necessary for systems that use select as the pollster,
such as Solaris (supported by gccgo).  It corresponds to the
bool returned by AddFD.  In general it's not clearly defined
what happens when a descriptor used in a select is closed, and
different systems behave differently.  Waking up the select
will cause the right thing to happen: the closed descriptor
will be dropped from the next iteration.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7303056