[nicklist] Improve terminal compatiblity of screen mode

[nicklist] Improve terminal compatiblity of screen mode

In screen mode, nicklist.pl uses the obscure escape sequences CSI s and
CSI u to save and restore the cursor position; on top of that, it uses
ESC P to bypass screen's interpretation of these escape sequences, passing
them directly to the underlying terminal.

While I did not investigate whether this makes sense at all - bypassing
screen's position tracking seems dangerous at best - this fix improves
terminal compatiblity by replacing CSI s and CSI u with the more common
ESC 7 and ESC 8 sequences [1].

The issue surfaced then vte3 replaced their parser, removing support for
CSI s and CSI u [2]. While support has been restored in the latest master
[3], using the more compatible escape sequences in nicklist.pl generally
seems like a good idea.

[1] https://gitlab.gnome.org/GNOME/vte/issues/48
[2] https://gitlab.gnome.org/GNOME/vte/issues/55
[3] https://gitlab.gnome.org/GNOME/vte/commit/f86cae603b23d34e36ab95fde6612bfc4d5eeca8