[cdc_rsync] Use ephemeral port on client (#96)

Instead of calling netstat locally to find out available ports in a
tight range, call bind() with port zero to find an available ephemeral
port. This is faster and much simpler, and will eventually help
getting rid of PortManager.

Also fixes issues with running SSH commands on Windows when the remote
shell is Powershell (aka Backslash Bingo).

cdc_rsync/BUILD

@@ -71,9 +71,9 @@ cc_library(
         "//common:path",
         "//common:path_filter",
         "//common:platform",
-        "//common:port_manager",
         "//common:process",
         "//common:remote_util",
+        "//common:server_socket",
         "//common:socket",
         "//common:status",
         "//common:status_macros",