os: reset dirinfo when seeking on Darwin

The first Readdirnames calls opendir and caches the result.
The behavior of that cached opendir result isn't specified on a seek
of the underlying fd. Free the opendir result on a seek so that
we'll allocate a new one the next time around.

Also fix wasm behavior in this regard, so that a seek to the
file start resets the Readdirnames position, regardless of platform.

p.s. I hate the Readdirnames API.

Fixes #35767.

Change-Id: Ieffb61b3c5cdd42591f69ab13f932003966f2297
Reviewed-on: https://go-review.googlesource.com/c/go/+/209961
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/dir_unix.go

@@ -26,6 +26,8 @@ const (
 
 func (d *dirInfo) close() {}
 
+func (f *File) seekInvalidate() {}
+
 func (f *File) readdirnames(n int) (names []string, err error) {
 	// If this file has no dirinfo, create one.
 	if f.dirinfo == nil {