[cdc_rsync] Enable local syncing (#75)

Adds support for local syncs of files and folders on the same Windows
machine, e.g. cdc_rsync C:\source C:\dest. The two main changes are

- Skip the check whether the port is available remotely with PortManager.
- Do not deploy cdc_rsync_server.
- Run cdc_rsync_server directly, not through an SSH tunnel.

The current implementation is not optimal as it starts
cdc_rsync_server as a separate process and communicates to it via a
TCP port.

common/path.cc

@@ -291,8 +291,8 @@ std::string GetDrivePrefix(const std::string& path) {
 
   if (path[0] != '\\') {
     size_t pos = path.find(":");
-    if (pos == std::string::npos) {
-      // E.g. "\path\to\file" or "path\to\file".
+    if (pos != 1) {
+      // E.g. "\path\to\file", "path\to\file" or "user@host:file".
       return std::string();
     }