We were flip-flopping between the two terms, so we now use one. We
still mention "array" in the chapter, and it's still `read --array`,
though.
Fixes#5846.
[ci skip]
As a simple replacement for `wc -l`.
This counts both lines on stdin _and_ arguments.
So if "file" has three lines, then `count a b c < file` will print 6.
And since it counts newlines, like wc, `echo -n foo | count` prints 0.
