cmd/go/internal/lockedfile, os: fix O_CREATE flag on Plan 9
os.OpenFile was assuming that a failed syscall.Open means the file does
not exist and it tries to create it. However, syscall.Open may have
failed for some other reason, such as failing to lock a os.ModeExclusive
file. We change os.OpenFile to only create the file if the error
indicates that the file doesn't exist.
Remove skip of TestTransform test, which was failing because sometimes
syscall.Open would fail due to the file being locked, but the
syscall.Create would succeed because the file is no longer locked. The
create was truncating the file.