Rename clear_screen() to input_clear_screen()

This matches input_unclear_screen(), and avoids a clash with the clear_screen identifier defined in
terminfo's <term.h>.

source/input.c

@@ -1300,10 +1300,10 @@ BUILT_IN_KEYBINDING(type_text)
 }
 
 /*
- * clear_screen: the CLEAR_SCREEN function for BIND.  Clears the screen and
+ * input_clear_screen: the CLEAR_SCREEN function for BIND.  Clears the screen and
  * starts it if it is held 
  */
-BUILT_IN_KEYBINDING(clear_screen)
+BUILT_IN_KEYBINDING(input_clear_screen)
 {
 	set_hold_mode(NULL, OFF, 1);
 	clear_window_by_refnum(0);

source/keys.c

@@ -190,7 +190,7 @@ static KeyMapNames 	key_names[] =
 #ifdef WANT_CHELP
 	{ "CHELP",			do_chelp },
 #endif
-	{ "CLEAR_SCREEN",		clear_screen 			},
+	{ "CLEAR_SCREEN",		input_clear_screen 			},
 	{ "COMMAND_COMPLETION",		command_completion 		},
 	{ "CPU_SAVER",			cpu_saver_on 			},
 	{ "DCC_PLIST",			dcc_plist },