Core: added signal name to "signal process started" logging

[Maxim Dounin]

Hello!

On Thu, Dec 18, 2014 at 12:05:01PM +0800, Xiaochen Wang wrote:

> It is useful to know which signal was sent to nginx master when we analyzed
> error.log.  If error_log level is warn/error/crit/alert/emerg, we could not
> know this information from nginx master/worker error log.

Adding more logging into signal process won't help, as signal 
process is not the only way to send signals to nginx.  The main 
purpose of signal process is to allow sending signals on Windows, 
where there is no "kill" to do this.

-- 
Maxim Dounin
http://nginx.org/