This frees a byte value to use for 'end of key' (to correctly sort
short keys) while still having a weight assigned to the field sep.
(Unless -t is given, the field sep is in the field data.)
Do reverse sorts by writing the output file in reverse order (rather
than reversing the sort - apart from merges).
All key compares are now unweighted.
For 'sort -u' mark duplicates keys during the sort and don't write
to the output.
Use -S to mean a posix sort - where equal keys are sorted using the
raw record (rather than being kept in the original order).
For 'sort -f' (no keys) generate a key of the folded data (as for -n
-i and -d), simplifies the code and allows a 'posix' sort.
Currently unchanged apart from the deletion of the 'unstable' version and
other unneeded code.
Use fldtab[0]. not fldtab-> when we are referring to the global info
in the 0th entry to emphasise that this entry is different.
fldtab[0].weights is only needed in the SINGL_FLD case - so set it there.
Re-indent a big 'if' is setfield() so that the line breaks match the
logic - which looks dubious now!
