prompt-util: add helpers that paint some "chrome" on top/bottom of screen

prompt-util: add helpers that paint some "chrome" on top/bottom of screen

We'll soon have three different kind of interactive "wizard"-like console
UIs: systemd-firstboot, homectl firstboot and soon systemd-sysinstall.
Let's give them a limited, recognizable visual identity, to distinguish
them from the usual console output: let's add a bit of "chrome" to the
top and bottom of the screen, that we show during ther wizards, but hide
again afterwards.

This makes use of the DECSTBM sequence that reduces the scrolling area
by chopping off blocks from the top or bottom of the screen. The
sequence is quite standard, given it has been part of VT100 already.
xterm, vte, Linux console all support it just fine.