big rework for dynamic flow definition

Pipes now accept updates of the input flow definition via upipe_set_flow_def.
A new uprobe_output probe, and inline functions upipe_void_alloc_output and
upipe_flow_alloc_output allow to transparently forward the flow definition
updates from pipe to pipe.

Other changes include renaming functions dealing with sample aspect ratio
attributes from "aspect" to "sar", and end-to-end latency calculation
using the latency attribute in the flow definition.