Make std.syslog() work from vcl_{init,fini}

Related to #1924.

bin/varnishtest/tests/r01924.vtc

-varnishtest "Test std.log from vcl_{init,fini}"
+varnishtest "Test std.{log,syslog} from vcl_{init,fini}"
 
 server s1 {
 	rxreq
@@ -10,10 +10,12 @@ varnish v1 -vcl+backend {
 
 	sub vcl_init {
 		std.log("init");
+		std.syslog(8 + 7, "init");
 	}
 
 	sub vcl_fini {
 		std.log("fini");
+		std.syslog(8 + 7, "fini");
 	}
 } -start
 

configure.ac

@@ -243,6 +243,7 @@ AC_CHECK_FUNCS([nanosleep])
 AC_CHECK_FUNCS([setppriv])
 AC_CHECK_FUNCS([fallocate])
 AC_CHECK_FUNCS([closefrom])
+AC_CHECK_FUNCS([vsyslog])
 
 save_LIBS="${LIBS}"
 LIBS="${PTHREAD_LIBS}"

lib/libvmod_std/vmod_std.c

@@ -163,14 +163,17 @@ vmod_syslog(VRT_CTX, VCL_INT fac, const char *fmt, ...)
 	txt t;
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	va_start(ap, fmt);
+	if (ctx->ws != NULL) {
 		u = WS_Reserve(ctx->ws, 0);
 		t.b = ctx->ws->f;
-	va_start(ap, fmt);
 		t.e = VRT_StringList(ctx->ws->f, u, fmt, ap);
-	va_end(ap);
 		if (t.e != NULL)
 			syslog((int)fac, "%s", t.b);
 		WS_Release(ctx->ws, 0);
+	} else
+		vsyslog((int)fac, fmt, ap);
+	va_end(ap);
 }
 
 VCL_VOID __match_proto__(td_std_collect)