su_port.h, su_port.c: added su_port_yield(),
refactored su_port_run() and su_port_step(),
moved su_wait() and call to event callback in su_port_wait_events().

Based on su_yield()/su_port_wiat_events() patch contributed by Colin
Whittaker.

darcs-hash:20060724182340-65a35-99a5cfa9d2b98afd81bd64fb73be89a39ca2fe39.gz