We should not require both files to be present for cmp(1)

If the second file is not present, read from standard input.
Make code a bit more clear
We do symbolic options for chmod(1)

Don't bomb out if the argument is not a valid octal number, it
could very well be something like "chmod +x test".
kill: argc < 2 should be argc < 1

Signed-off-by: Hiltjo Posthuma <hiltjo@codemadness.org>
Rename mc(1) to cols(1)

There is an obvious name collision with the popular file manager
midnight commander.