lualoader: Add a twiddle at password prompt

This gives some form of feedback while typing, and matches-(ish*) Forth
behavior. The cursor generally rests two column after the password prompt,
then the twiddle is drawn three columns later and the cursor reset to
resting position after being drawn.

I've removed the note about re-evaluating it for security considerations and
instead set it up as a module-local variable that we can set later depending
on environment or something. It's set to false with no chance of changing at
the moment.

*As close as I can tell from reading check-password.4th, because I don't
have an easy test (or deployed) setup for forth loader to check how close
it is. Please do mention if it's not close enough.

stand/lua/password.lua

@@ -33,27 +33,46 @@ local core = require("core")
 local screen = require("screen")
 
 local password = {}
+-- Asterisks as a password mask
+local show_password_mask = false
+local twiddle_chars = {"/", "-", "\\", "|"}
+local twiddle_pos = 1
 
 -- Module exports
 function password.read()
 	local str = ""
 	local n = 0
 
+	twiddle_pos = 1
+	local function draw_twiddle()
+		loader.printc("  " .. twiddle_chars[twiddle_pos])
+		screen.movecursor(-3, -1)
+		twiddle_pos = (twiddle_pos % #twiddle_chars) + 1
+	end
+
+	-- Space between the prompt and any on-screen feedback
+	loader.printc(" ")
 	while true do
 		local ch = io.getchar()
 		if ch == core.KEY_ENTER then
 			break
 		end
-		-- XXX TODO: Evaluate if we really want this or not, as a
-		-- security consideration of sorts
 		if ch == core.KEY_BACKSPACE or ch == core.KEY_DELETE then
 			if n > 0 then
 				n = n - 1
-				-- loader.printc("\008 \008")
+				if show_password_mask then
+					loader.printc("\008 \008")
+				else
+					draw_twiddle()
+				end
 				str = str:sub(1, n)
 			end
 		else
-			-- loader.printc("*")
+			if show_password_mask then
+				loader.printc("*")
+			else
+				draw_twiddle()
+			end
 			str = str .. string.char(ch)
 			n = n + 1
 		end

stand/lua/screen.lua

@@ -49,6 +49,24 @@ function screen.setcursor(x, y)
 	loader.printc("\027[" .. y .. ";" .. x .. "H")
 end
 
+function screen.movecursor(dx, dy)
+	if core.isSerialBoot() then
+		return
+	end
+
+	if dx < 0 then
+		loader.printc("\027[" .. -dx .. "D")
+	elseif dx > 0 then
+		loader.printc("\027[" .. dx .. "C")
+	end
+
+	if dy < 0 then
+		loader.printc("\027[" .. -dy .. "A")
+	elseif dy > 0 then
+		loader.printc("\027[" .. dy .. "B")
+	end
+end
+
 function screen.setforeground(color_value)
 	if color.disabled then
 		return color_value