[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_test.cc

@@ -302,6 +302,7 @@ TEST_F(PathTest, GetDrivePrefix) {
   EXPECT_EQ(path::GetDrivePrefix("C:\\"), "C:");
   EXPECT_EQ(path::GetDrivePrefix("C:\\dir"), "C:");
   EXPECT_EQ(path::GetDrivePrefix("C:\\dir\\file"), "C:");
+  EXPECT_EQ(path::GetDrivePrefix("host:C:\\dir\\file"), "");
 }
 #endif