Add parallel transaction support for remote source or target specifications.

Add parallel transaction support for remote source or target specifications.
The implementation is a bit crude because I don't want to take too many
chances on a codebase that wasn't originally designed to be multi-threaded,
so the master mutex is only released when a thread is waiting for input
on a socket.

* Add the -p<threads> option and compile -pthread by default.  This is only
  useful when the source and/or destination is a remote host.  Note that
  parallel transaction mode will not work with older cpdup binaries on the
  remote end.

  This greatly improves cpdup's performance when operating on a remote
  source and/or target.

* Add -l to force stdout and stderr to be line-buffered.