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.
CDC RSync is a command line tool / library for uploading files to a remote machine in an rsync-like
fashion. It quickly skips files with matching timestamp and size, and only transfers deltas for
existing files.