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

[Xiaochen Wang]

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.

The error log with this patch is changed as following:
2014/12/18 11:47:42 [notice] 17320#0: signal process started (reopen)
2014/12/18 11:47:44 [notice] 17346#0: signal process started (reload)

# HG changeset patch
# User Xiaochen Wang <wangxiaochen0 at gmail.com>
# Date 1418872566 -28800
# Node ID 179a8ce4f68f96c316e4daec3fa677cd7d4e0126
# Parent  99751fe3bc3b285801b434f7f707d87fa42b093e
Core: added signal name to "signal process started" logging

diff -r 99751fe3bc3b -r 179a8ce4f68f src/core/ngx_cycle.c
--- a/src/core/ngx_cycle.c	Fri Dec 12 20:25:42 2014 +0300
+++ b/src/core/ngx_cycle.c	Thu Dec 18 11:16:06 2014 +0800
@@ -969,7 +969,8 @@ ngx_signal_process(ngx_cycle_t *cycle, c
     ngx_core_conf_t  *ccf;
     u_char            buf[NGX_INT64_LEN + 2];
 
-    ngx_log_error(NGX_LOG_NOTICE, cycle->log, 0, "signal process started");
+    ngx_log_error(NGX_LOG_NOTICE, cycle->log, 0,
+                  "signal process started (%s)", sig);
 
     ccf = (ngx_core_conf_t *) ngx_get_conf(cycle->conf_ctx, ngx_core_module);