Add event_enable_debug_logging() to control use of debug logs

Add event_enable_debug_logging() to control use of debug logs

Previously, debug logs were turned on if you built with -DUSE_DEBUG
and off otherwise.  This make builds with -DUSE_DEBUG hideously slow
and other builds unable to get debug logs.

This is based off a patch by Ralph Castain from October.  It tries a
little harder to avoid needless function calls, it doesn't require
stdbool, and makes the controlling parameter a mask rather than a
boolean so that we can later support enabling only the debugging
messages for the parts of Libevent you're trying to debug.