syscall,os,net: don't use ForkLock in plan9

syscall,os,net: don't use ForkLock in plan9

This is the follow-on to CL 22610: now that it's the child instead of
the parent which lists unwanted fds to close in syscall.StartProcess,
plan9 no longer needs the ForkLock to protect the list from changing.
The readdupdevice function is also now unused and can be removed.

Change-Id: I904c8bbf5dbaa7022b0f1a1de0862cd3064ca8c7
Reviewed-on: https://go-review.googlesource.com/22842
Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>