ptyfwd: when leaving a session with tinted background, clear to end of screen

So if we tint the background of a ptyfwd session with a color and the
session ends, then so far we reset the bg color and clear till the end
of line.

Let's instead clear till the end of the screen. This is nicer since it
means that any follow-up output will not be affected by the changed
background color anymore.

src/basic/terminal-util.h

@@ -80,6 +80,9 @@
 /* Erase characters until the end of the line */
 #define ANSI_ERASE_TO_END_OF_LINE "\x1B[K"
 
+/* Erase characters until end of screen */
+#define ANSI_ERASE_TO_END_OF_SCREEN "\x1B[J"
+
 /* Move cursor up one line */
 #define ANSI_REVERSE_LINEFEED "\x1BM"