runtime: use gList in netpoll

netpoll is perhaps one of the most confusing uses of G lists currently
since it passes around many lists as bare *g values right now.
Switching to gList makes it much clearer what's an individual g and
what's a list.

Change-Id: I8d8993c4967c5bae049c7a094aad3a657928ba6c
Reviewed-on: https://go-review.googlesource.com/129397


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>