Xorg: honor AutoRepeat option

This option was implemented before the drivers were split in ≈2006,
and e.g. XWin still supports it.

With this commit, Xorg regains support, so that the following configuration can
be used to set the repeat rate for all keyboard devices without having to modify
Xorg command-line flags or having to automate xset(1):

Section "InputClass"
        Identifier "system-keyboard"
        MatchIsKeyboard "on"
        Option "XkbLayout" "de"
        Option "XkbVariant" "neo"
	Option "AutoRepeat" "250 30"
EndSection

Signed-off-by: Michael Stapelberg <stapelberg@google.com>
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