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.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Bump shared libraries version numbers in preparation for 9.0.
This time, only libraries which ABI has been changed compared to
stable/8, are bumped.

ABI analysis done by: Gleb Kurtsou
Approved by: re (kensmith)

Spelling fixes.

mdoc policing: use consistent section ordering, fix quotes and trailing
whitespace

Approved by: rpaulo

Import the librtld_db library. This is needed by userland DTrace.
This is not yet enabled in the build because I also need to import a new
version of libproc.

Sponsored by: The FreeBSD Foundation