Fix trust-sigil handling when scrolling the terminal.

Previously, when we scrolled the terminal, the newly exposed line at
the bottom would be immediately allocated a trust status corresponding
to the current state of the terminal. So if you're in trusted mode and
you print a newline, then the line scrolled on at the bottom
immediately gets a trust sigil, whether you subsequently print
anything on it or not.

Up until now, that hasn't mattered, because we always _do_ print
something on it. But if you don't - if you send \r\n\r\n to
deliberately leave a blank line - then it turns out that's not what we
want after all, because if the screen _doesn't_ scroll, the
passed-over line remains completely blank, whereas if it does scroll
the blank line gets a trust sigil, which is inconsistent.

Now, terminal lines newly exposed by a scroll have untrusted status,
just the same as terminal lines that were present in the initial blank
screen. They only become trusted if you actually print at least one
character on them (whereupon check_trust_status will re-clear them
just in case). And this is now independent of whether the terminal has
scrolled or not.

terminal/terminal.c

@@ -2618,7 +2618,7 @@ static void scroll(Terminal *term, int topline, int botline,
             }
             resizeline(term, line, term->cols);
             clear_line(term, line);
-            check_trust_status(term, line);
+            line->trusted = false;
             addpos234(term->screen, line, botline);
 
             /*