[release-branch.go1.21] os: RemoveAll: fix symlink race for unixrelease-branch.go1.21

Since all the platforms now support O_DIRECTORY flag for open, it can be
used to (together with O_NOFOLLOW) to ensure we open a directory, thus
eliminating the need to call stat before open. This fixes the symlink race,
when a directory is replaced by a symlink in between stat and open calls.

While at it, rename openFdAt to openDirAt, because this function is (and was)
meant for directories only.

NOTE Solaris supports O_DIRECTORY since before Solaris 11 (which is the
only version Go supports since supported version now), and Illumos
always had it. The only missing piece was O_DIRECTORY flag value, which
is taken from golang.org/x/sys/unix.

Fixes #67695.

Change-Id: Ic1111d688eebc8804a87d39d3261c2a6eb33f176
Reviewed-on: https://go-review.googlesource.com/c/go/+/589057
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

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