Syslog: enabled logging of send errors.

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.
Syslog: improved error handling of unix domain sockets.

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.