refclock: split off median filter

Move the implementation of the median filter to a separate file to make
it useful for NTP. Replace some constants with parameters and generalize
the code to work with full NTP samples (including root dispersion/delay,
stratum, and leap).

For refclocks it should give the same results as before.

Makefile.in

@@ -36,7 +36,7 @@ DESTDIR=
 HASH_OBJ = @HASH_OBJ@
 
 OBJS = array.o cmdparse.o conf.o local.o logging.o main.o memory.o \
-       reference.o regress.o rtc.o sched.o sources.o sourcestats.o stubs.o \
+       reference.o regress.o rtc.o samplefilt.o sched.o sources.o sourcestats.o stubs.o \
        smooth.o sys.o sys_null.o tempcomp.o util.o $(HASH_OBJ)
 
 EXTRA_OBJS=@EXTRA_OBJECTS@