nameserv: prepare for asynchronous resolving

Introduce a new function with callback to resolve names to IP addresses
asynchronously. For now, use a blocking wrapper around DNS_Name2IPAddress.

nameserv_async.h

@@ -0,0 +1,41 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2014
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Header for asynchronous nameserver functions
+  */
+
+
+#ifndef GOT_NAMESERV_ASYNC_H
+#define GOT_NAMESERV_ASYNC_H
+
+#include "nameserv.h"
+
+/* Function type for callback to process the result */
+typedef void (*DNS_NameResolveHandler)(DNS_Status status, IPAddr *ip_addr, void *anything);
+
+/* Request resolving of a name to IP address. The handler will be
+   called when the result is available, but it may be also called
+   directly from this function call. */
+extern void DNS_Name2IPAddressAsync(const char *name, DNS_NameResolveHandler handler, void *anything);
+
+#endif