Reviving earlier work by @ality in https://golang.org/cl/57890043
to make the closing of extra file descriptors in syscall.StartProcess
less race-prone. Instead of making a list of open fds in the parent
before forking, the child can read through the list of open fds and
close the ones not explicitly requested.  Also eliminate the
complication of keeping open any extra fds which were inherited by
the parent when it started.

This CL will be followed by one to eliminate the ForkLock in plan9,
which is now redundant.

Fixes #5605

Change-Id: I6b4b942001baa54248b656c52dced3b62021c486
Reviewed-on: https://go-review.googlesource.com/22610
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: David du Colombier <0intro@gmail.com>