A lot of terminals support CSI Ps S. Currently we only allow them
to use scrollback-up if they advertise it via XTGETTCAP. This seems
surprising; it's better to make visible in fish script whether this
is supposed to be working. The canonical place is in "bind ctrl-l"
output.
The downside here is that we need to expose something that's rarely
useful. But the namespace pollution is not so bad, and this gives
users a nice paper trail instead of having to look in the source code.
b964072c11