winterm: fix GetStdFile() falltrough

[thaJeztah]

Commit 3ee0038 (#31) updated the switch with the intent to handle either syscall.XX or golang.org/x/sys/win.XX values.

However, Go "switch" does not have an implicit "fallthrough" behavior, and either requires the "fallthrough" keyword to be used, or to combine values in a single "case".

For example, the following:

for _, v := range []int{1, 2, 3, 4, 5, 6, 7} {
    result := "unknown"
    switch v {
    case 1: // doesn't fall-through, so this is an empty "case"
    case 2:
        result = "found one or two"
    case 3:
        fallthrough // falls through to "case 4"
    case 4:
        result = "found three or four"
    case 5, 6: // matches either 5 or 6
        result = "found five or six"
    default:
        result = "found something else"
    }
    fmt.Printf("%d: %s\n", v, result)
}

Will print:

1: unknown
2: found one or two
3: found three or four
4: found three or four
5: found five or six
6: found five or six
7: found something else

This patch combines "equal" values in a single case within the switch to
fix this issue.

[thaJeztah]

@jstarks @n4j PTAL

[jstarks]

Whoops. I should have looked more closely at the original change. Thanks, LGTM.

[thaJeztah]

Definitely easy to miss, as other languages do fall through, and it doesn't look "wrong"

Thanks for the merge!