If a syslog daemon is restarted and the unix socket is used, further logging
might stop to work.  In case of send error, socket is closed, forcing
a reconnection at the next logging attempt.

The ngx_cycle->log is used when sending the message.  This allows to log syslog
send errors in another log.

Logging to syslog after its cleanup handler has been executed was prohibited.
Previously, this was possible from ngx_destroy_pool(), which resulted in error
messages caused by attempts to write into the closed socket.

The "processing" flag is renamed to "busy" to better match its semantics.

Now we actually return NGX_ERROR on errors, and provide an error
string for memory allocation errors.

Reported by Markus Linnala.

In theory, this can provide a bit better distribution of latencies.

Also it simplifies the code, since ngx_queue_t is now used instead
of custom implementation.

It's mostly dead code.  And the idea of thread support for this task has
been deprecated.

If a "resolver_timeout" occurs, only the first waiting request
was notified.  Other requests may hang forever.