Fix unused argument warning.
Fix sprintf format warning in example.
Remove unused data member.
Use a single condition variable per io_service.

This change fixes a potential race condition when using run_one() from
multiple threads. It also paves the way for the future addition of
timed io_service::run() overloads.
Use scope id as default IPv6 multicast interface.
Support stringified scope ids for link-local multicast.
Use link-local multicast on Mac OS.
Don't run the compile-time buffer test.

Fixes boost trac ticket #8295.
Clarify that buffer_copy does not support overlapping memory regions.

Fixes boost trac ticket #8295.
Regenerate documentation.