Notes on IO::Filter::External
-----------------------------

"w" -- write mode
-----------------

Sets up a pipeline like this:

  your program               		external                $io acting
  writing to    ---------->  buffer -->	program   ----------->  as the sink
  $fio                                  		        of data

Each syswrite causes:

	syswrite to write pipe (won't ever block, obviously)
	keep reading from external program and writing to $io until blocks

Close causes:

	close write pipe
	keep reading from external program and writing to $io until EOF
	close read pipe
	close $io

"r" -- read mode
----------------

  your program               external                $io acting
  reading from  <----------  program   <-----------  as the source
  $fio                                               of data

Each sysread causes:

	loop forever:
		select on read from $io and read from read pipe
		if (read from $io)
			read from $io and write to write pipe
		if (read from read pipe)
			return data to user

Close causes:

	close everything (?)