redmine6466: preserve order in readstringarrayidx

Reading the file's contents via a Set is harmless for the other
read*array functions, since they use data from each line as the key,
so naturally discard duplicates and have no other ordering than that
of the keys themselves; however, for readstringarrayidx, the order of
lines actually matters and duplicates are meaningful.  So convert
BuildLineArray to use an Rlist instead of a StringSet (and iterator);
and add an acceptance test (which was actually the hard part).