Replace ukbhit by kbd_enter_pressed, not requiring tcgetattr:

Note that it behaves differently now
* it looks for Enter key only, not any key
* it "eats" the input, no need for (void)getchar() after it
* it works the same no matter the value of ICANON
* the mingw version has been adapted to act the similarly

This should fix its usage on Android where tcgetattr always returns -1

client/scripts/mfkeys.lua

@@ -208,7 +208,7 @@ local function perform_check(numsectors)
 
     for sector = 0, #keys do
         -- Check if user aborted
-        if core.ukbhit() then
+        if core.kbd_enter_pressed() then
             print('Aborted by user')
             break
         end