Debug multipipe block to mutipipe block connections

[mfragkoulis]

Graphs that include multipipe block to mutipipe block connections, such as the following one, hang at the negotiation phase.

{{
echo hi &
echo there &
echo stranger &
}} |
{{
cat -n &
dd &
}}

[dspinellis]

Suggestion: when producers or consumers with a flexible input / output channel number appear use the smallest number of channels possible, equally dividing the remaining channels among processes, favouring the top ones for the remainder.