Valgrind: sigaction() failure now ignored.

Valgrind intercepts SIGUSR2 in some cases, and nginx might not be able to
start due to sigaction() failure.  If compiled with NGX_VALGRIND defined,
we now ignore the failure of sigaction().

src/os/unix/ngx_process.c

diff --git a/src/os/unix/ngx_process.c b/src/os/unix/ngx_process.c
index 4ef3582..6f3f385 100644
--- a/src/os/unix/ngx_process.c
+++ b/src/os/unix/ngx_process.c
@@ -291,9 +291,14 @@
         sa.sa_handler = sig->handler;
         sigemptyset(&sa.sa_mask);
         if (sigaction(sig->signo, &sa, NULL) == -1) {
+#if (NGX_VALGRIND)
+            ngx_log_error(NGX_LOG_ALERT, log, ngx_errno,
+                          "sigaction(%s) failed, ignored", sig->signame);
+#else
             ngx_log_error(NGX_LOG_EMERG, log, ngx_errno,
                           "sigaction(%s) failed", sig->signame);
             return NGX_ERROR;
+#endif
         }
     }