Doesn't really matter for the most part, since the runtime-integrated
network poller uses its own kevent implementation, but for people using
the syscall directly, we should use an unsafe.Pointer for the precise GC
to retain the pointer arguments.

Also push down unsafe.Pointer a bit further in exec_linux.go, not
that there are any GC preemption points in the middle and sys
is still live anyway.

R=golang-codereviews, dvyukov
CC=golang-codereviews, iant
https://golang.org/cl/55520043