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.go

@@ -0,0 +1,12 @@
+// +build !windows
+
+package fs
+
+// isDriveLetter returns a bool indicating whether name is a valid
+// Windows drive letter
+//
+// On non windows platforms we don't have drive letters so we always
+// return false
+func isDriveLetter(name string) bool {
+	return false
+}