windows: Stop drive letters (eg C:) getting mixed up with remotes (eg drive:)

This was done by stopping the user configuring single letter remotes
and making sure we don't treat single letter remotes as a remote name,
but as a drive letter.

fs/driveletter_windows.go

@@ -0,0 +1,13 @@
+// +build windows
+
+package fs
+
+// isDriveLetter returns a bool indicating whether name is a valid
+// Windows drive letter
+func isDriveLetter(name string) bool {
+	if len(name) != 1 {
+		return false
+	}
+	c := name[0]
+	return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')
+}