Fix regression preserving tcsh command prompt
Commit c3b3ef19 introduced a regression when invoking the clear-history-and-reset action. While RIS (Reset to Initial State) is specified in DEC STD-070 as homing the cursor, konsole has grown some hacks in the name of usability to preserve the command prompt line.
For a long time, it has sent two SIGWINCH with changed sizes after clear and reset actions to force the shell to redraw the prompt (see d346a2cc, temporarily disabled on 5d61b69e and re-added on 82778e87), which works for bash, zsh, ksh, ...
tcsh doesn't redraw its prompt on SIGWINCH, but commit b8e96bcd modified Screen::refresh() so instead of clearing the entire screen and homing the cursor, it scrolled up everything but the last (usually the prompt) line.
So, keep that last hack when called from clear-history-and-reset, and behave as specified on DEC STD-070 otherwise.
Note that other ways of clearing the screen don't need hacks, e.g. Ctrl-L, if handled at all, is handled by the shell, which then redraws its prompt. Calling "clear" or invoking "printf '\ec'" will result in the shell redrawing its prompt in the usual way.