lualoader: Revert to ASCII menu frame for serial console

The box drawing characters we use aren't necessarily safe with a serial
console; for instance, in the report by npn@, these were causing his xterm
to send back a sequence that lua picked up as input and halted the boot.
This is less than ideal.

Fallback to ASCII frames for console with 'comconsole' in it.  This is a
partial revert r338108 by imp@ -- instead of removing the menu entirely and
disabling color/cursor sequences, just reverting the default frame to ASCII
is enough to not break in this setup.

Reported by:	npn
Triaged and recommended by:	tsoome

stand/lua/core.lua

@@ -325,6 +325,16 @@ function core.isZFSBoot()
 	return false
 end
 
+function core.isSerialConsole()
+	local c = loader.getenv("console")
+	if c ~= nil then
+		if c:find("comconsole") ~= nil then
+			return true
+		end
+	end
+	return false
+end
+
 function core.isSerialBoot()
 	local s = loader.getenv("boot_serial")
 	if s ~= nil then

stand/lua/drawer.lua

@@ -144,13 +144,20 @@ local function drawmenu(menudef)
 	return alias_table
 end
 
+local function defaultframe()
+	if core.isSerialConsole() then
+		return "ascii"
+	end
+	return "double"
+end
+
 local function drawbox()
 	local x = menu_position.x - 3
 	local y = menu_position.y - 1
 	local w = frame_size.w
 	local h = frame_size.h
 
-	local framestyle = loader.getenv("loader_menu_frame") or "double"
+	local framestyle = loader.getenv("loader_menu_frame") or defaultframe()
 	local framespec = drawer.frame_styles[framestyle]
 	-- If we don't have a framespec for the current frame style, just don't
 	-- draw a box.