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.

Makefile.in

@@ -42,7 +42,7 @@ OBJS = util.o sched.o regress.o local.o \
 	sys.o main.o ntp_io.o ntp_core.o ntp_sources.o \
 	sources.o sourcestats.o reference.o \
 	logging.o conf.o cmdmon.o keys.o \
-	nameserv.o manual.o addrfilt.o \
+	nameserv.o nameserv_async.o manual.o addrfilt.o \
 	cmdparse.o mkdirpp.o rtc.o pktlength.o clientlog.o \
 	broadcast.o refclock.o refclock_phc.o refclock_pps.o \
 	refclock_shm.o refclock_sock.o tempcomp.o $(HASH_OBJ)