loader: Use printc instead of print to fit in 24,80

print automatically adds a newline, while printc does not. Use printc in
preference to print for managing the autoboot message. This means we can
use line 24 safely on a 24x80 terminal, restoring some functionality
that was lost in 101afbc6ee2f0.

Note: we still set the default curosor position to 25,1 in screen.lua,
but real VT100s (and successors) will treat any row larger than the
pnumber of rows in a cursor motion command to be the last physical row
(this is so you can move to 9999,9999 and do a cursor location query to
get the size of the screen). Keeping that as is looks better on a
typical VGA console.

Fixes: 101afbc6ee2f0
Sponsored by: Netflix
Reviewed by: kevans
Differential Revision:	https://reviews.freebsd.org/D46771

(cherry picked from commit 3fb656f8ef21332d96de8097521aaa51ddeb649d)

stand/lua/menu.lua

@@ -530,6 +530,7 @@ function menu.run()
 	drawn_menu = nil
 
 	screen.defcursor()
+	-- We explicitly want the newline print adds
 	print("Exiting menu!")
 end
 
@@ -544,7 +545,7 @@ function menu.autoboot(delay)
 		if last == nil or last ~= time then
 			last = time
 			screen.setcursor(x, y)
-			print("Autoboot in " .. time ..
+			printc("Autoboot in " .. time ..
 			    " seconds. [Space] to pause ")
 			screen.defcursor()
 		end
@@ -553,9 +554,12 @@ function menu.autoboot(delay)
 			if ch == core.KEY_ENTER then
 				break
 			else
-				-- erase autoboot msg
+				-- Erase autoboot msg.  While real VT100s
+				-- wouldn't scroll when receiving a char with
+				-- the cursor at (24, 79), bad emulators do.
+				-- Avoid the issue by stopping at 79.
 				screen.setcursor(0, y)
-				print(string.rep(" ", 80))
+				printc(string.rep(" ", 79))
 				screen.defcursor()
 				return ch
 			end