os: try openFile before openDir in windows os.OpenFile
Logging calls when running "go install -a std" turns:
547 openDir succeeded
3593 openDir failed and fell back to openFile
3592 openFile succeeded
1 both failed
into:
3592 openFile succeeded
548 openFile failed and fell back
547 openDir succeeded
1 both failed
Here the change trades 3593 failed openDir for 548 failed openFile.
Fix issue 7426.
LGTM=alex.brainman
R=golang-codereviews, alex.brainman, bradfitz
CC=golang-codereviews
https://golang.org/cl/
70480044