Handle the different event types properly in rd_event_addr(). In particular,
with r265456 _r_debug_postinit can be used for RD_POSTINIT events. rtld(1)
uses r_debug_state for dl state transitions, so we use its address for
RD_DLACTIVITY events.

MFC after:	2 weeks
Fix the rd_event_addr prototype and slightly clarify the use of the "event"
parameter.

MFC after:	3 days