[common] Add support for ClientSocket on Linux (#97)

Uses the abstractions written for ServerSocket in ClientSocket, so
that it builds on Linux. Also adds a method to poll for connections
and uses that in cdc_rsync. Similar code will be used in cdc_fuse_fs
to wait for a connection in a future CL.

common/client_socket.h

@@ -29,6 +29,10 @@ class ClientSocket : public Socket {
   ClientSocket();
   ~ClientSocket();
 
+  // Polls until a connection to |port| succeeds.
+  // Returns TimeoutError
+  static absl::Status WaitForConnection(int port, absl::Duration timeout);
+
   // Connects to localhost on |port|.
   absl::Status Connect(int port);