Let packetizer handle 0-length sends from channel.

If the channel is closed the send method returs a response length
of 0. This is not handled correctly by the packetizer and puts it
in an infinite loop. (Fixes #156 for real :-)

We make sure we don't do more than 10 iteration on a 0 length respose,
but raise an EOFError.
Whitespace fixes.
Add new _send method to reduce code duplication.

This method is the lowest _send related method on Channel and is
the place where we raise a socket is closed Excpetion.
Use new _send method.
Remove raising of socket excpetions from sendall*.