Implement the new -D flag that allows you to specify a progress dialog app

Example usage:
-D 'bar -s %%size%% -bl 1m'

Current marcos are %%size%% (incremental dataset size in bytes) and
%%title%% (source dataset name)

dd output makes it a bit ugly, but apparently makes sure 'zfs send' is
never blocked by the pipe system call (see FreeNAS forums)