ntp: drop initstepslew NTP implementation

The initstepslew code has its own minimal NTP implementation. Drop the
code, add a new initstepslew mode to the reference updating code and
use regular NTP sources with iburst flag for initstepslew addresses
instead. When an update is made or a source is found unreachable, log a
message, remove the initstepslew sources and switch to normal mode.

This reduces code duplication and makes initstepslew use features
implemented only in the main code like source combining or SO_TIMESTAMP
support.

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 acquire.o manual.o addrfilt.o \
+	nameserv.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)