Clean up handling of median, by treating it almost like any other

Clean up handling of median, by treating it almost like any other
percentile.  Fixes bug #17424.  Thanks to John Darrington for
review.
* frequencies.q	(internal_cmd_frequencies): Fix handling of bit
masks for `stats' variable.  If median is selected, turn it off
and add a 50th percentile.
(add_percentile): Simplify code a little.
(calc_stats): Drop special casing of median.
(dump_statistics): Ditto, except that we label the 50th percentile
as "50 (Median)" to make it clear that it's also the median.