MFC r265629, r265630

MFC r265629:
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 r265630:
Fix the rd_event_addr prototype and slightly clarify the use of the "event"
parameter.
MFC r269470: Avoid divide-by-zero panic when setting baudrate to 0.