Sort lines in region lexicographically by the ARGth field of each line.
Fields are separated by whitespace and numbered from 1 up.
With a negative arg, sorts by the ARGth field counted from the right.
Called from a program, there are three arguments:
FIELD, BEG and END. BEG and END specify region to sort.
The variable `sort-fold-case' determines whether alphabetic case affects
the sort order.
(fn FIELD BEG END)