This ensures that errors which happen during logging to syslog are logged
with proper context, such as "while logging to syslog" and the server name.

Prodded by Safar Safarly.

During initial startup the ngx_cycle->hostname is not available, and
previously this resulted in incorrect logging.  Instead, hostname from the
configuration being parsed is now preserved in the syslog peer structure
and then used during logging.

Similarly, ngx_cycle->log might not match the configuration where the
syslog peer is defined if the configuration is not yet fully applied,
and previously this resulted in unexpected logging of syslog errors
and debug information.  Instead, cf->cycle->new_log is now referenced
in the syslog peer structure and used for logging, similarly to how it
is done in other modules.

If a socket was re-opened due to an error (02c2352d5b01 and fa0e093b64d7),
additional cleanup handler was installed each time.

The option disables sending hostname in the syslog message header.  This is
useful with syslog daemons that do not expect it (tickets #677 and #783).

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.