Ctrl+Alt+Backspace is usually enabled by default under X, and is a
keyboard shortcut that forcefully kills the current X server. This can
lead to data loss by users if accidentally pressed. This commit
introduces a new option, services.xserver.enableCtrlAltBackspace, that
is *disabled* by default. If set to true, the previous behavior can be
restored.
A similar decision was made by the Ubuntu team, and is documented here:
https://wiki.ubuntu.com/XorgCtrlAltBackspace
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