comm: ensure that input files are sorted

comm: ensure that input files are sorted

* NEWS: List new behavior.
* doc/coreutils.texi (checkOrderOption) New macro for
describing `--check-order' and `--nocheck-order', used in
both join and comm.
* src/comm.c (main): Initialize new options.
(usage): Describe new options.
(compare_files): Keep an extra pair of buffers for the previous
line from each file to check the internal order.
(check_order): If an order-check is required, compare and handle
the result appropriately.
(copylinebuffer): Copy a linebuffer; used for copy before read.
* tests/misc/Makefile.am: List new test.
* tests/misc/comm: Tests for the comm program, including the
new order-checking functionality and attendant command-line options.