ngx_lua support syslog which is added since nginx 1.7.1

syslog was supported officially since nginx 1.7.1, the two new members `writer` and `wdata` in struct `ngx_log` were added for this feature. The two member need to be set when initialize logger int ngx_lua, or nginx worker will crash with signal 11. 

Here is the testcase:
<pre>
error_log  syslog:server=127.0.0.1,facility=local1,tag=nginx info;
...
http {
    ...
    init_worker_by_lua 'ngx.log(ngx.ERR, "crash"); ngx.timer.at(0, function() ngx.log(ngx.ERR, "crash") end);';
}
</pre>