since commit 8c16ac47c1b88fc48ad8ea621a75a86e028a2764, we started
creating the login screen on display 1024 instead of display 0.

This defeats this logic in try_display:

    display++;
    /* If display is above 50, then something's wrong. Just
     * abort in this case. */
    if (display > 50)

In practice it doesn't matter much since we only have one login
screen in most setups, but we should still fix the bug.

This commit introduces a separate counter to keep try of 50 tries,
rather than assuming "display number == number of tries".