add a new hook iomux_loop_next_cb() to allow having a callback called at each runcycle

just before calling iomux_run(), opposed to iomux_loop_end_cb() which is instead called
before iomux_loop() returns, after the runloop has been already interrupted.

The hangup hook stays because that's the one supposed to break the runloop preventing a next
call to iomux_run().

+ documentation + tests