Change the multilog code to toggle between async and sync writes for all

Change the multilog code to toggle between async and sync writes for all
log types.  This means the threaded_syslog type is no longer valid.  A new
fxn multilog_async is available to toggle between the two modes.  If an
app is compiled without pthreads and tries to use async logging, no logging
will occur while async is enabled.

dmeventd has been modified to use the new code

I'm not positive I like the way the async_logger code calls the log fxn,
but it works for now.  Suggestions for other ways to do it would be helpful