os: disallow Root.Remove(".") on Plan 9, js, and Windows

Windows already forbids this, since removing the root causes a
sharing violation (can't delete the directory while the os.Root
has a handle open to it), but add a more explicit check for
attempts to delete "." and return EINVAL.

Note that this change to Windows doesn't affect operations like
Root.Remove("dir/."), since the path is cleaned into just "dir"
before attempting the deletion.

Fixes #73863

Change-Id: I0f45ccb6c9f171d3a52831632c134150388d77b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/679377
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions