better handling of blocking simcalls in the generated popping

Blocking simcalls were marked as simcalls returning void. Being
explicit helps in that obscure part of the code.
rename smx_action_t to smx_synchro_t

This is an old idea, that I want to implement since at least 3 years.
It removes an ambiguity with surf_action_t, and makes it explicit that
these things are a way to synchronize the processes with their
environment.