[Bug-ed] Scrolls N lines at a time... adjustments

[積丹尼 Dan Jacobson]

We read
'(.+1)z N'
     Scrolls N lines at a time starting at addressed line. If N is not
     specified, then the current window size is used. The current
     address is set to the last line printed.
also 'n' does the same.

But where does it get the current window size?

$ infocmp
#       Reconstructed via infocmp from file: /lib/terminfo/d/dumb
dumb|80-column dumb tty,
        am,
        cols#80,
        bel=^G, cr=^M, cud1=^J, ind=^J,

Well apparently there is a default of 24.

So document it, and give an easy way to change it via e.g., $LINES.

$ ed --version
GNU Ed 1.10