sort: add -k, fix -u

Options  that are  specific to  a  single  key  definition are not
supported (e.g. "sort -k 2,3n -k 4,4").  Should you try to specify
such definitions, sort will  return with EXIT_FAILURE and an error
message.   Instead, all key definitions exclusively use the global
settings.

It always behaves like -b was set.

sort.1

@@ -4,6 +4,9 @@ sort \- sort lines
 .SH SYNOPSIS
 .B sort
 .RB [ \-nru ]
+.RB [ \-k
+.I key
+.R ]...
 .RI [ file ...]
 .SH DESCRIPTION
 .B sort
@@ -18,4 +21,18 @@ perform a numeric sort.
 reverses the sort.
 .TP
 .B \-u
-prints repeated lines only once.
+prints equal lines only once.
+.TP
+.B \-k key
+specifies a key definition of the form \fBS\fR[.\fBs\fR][,\fBE\fR[.\fBe\fR]],
+where
+.B S,
+.B s,
+.B E,
+and
+.B e
+are the starting column, starting character in that column, ending column and
+the ending character of that column respectively.  If they are not specified,
+s refers to the first character of the specified starting column, E refers to
+the last column of every line, and e refers to the last character of that last
+column.