System Preferences -> Keyboard -> Keyboard Shortcuts
And change the radio at the bottom to "All controls" under Full Keyboard Access.
In the System Preferences window, you can start typing to select 'Keyboard' (keyboard focus is in the Spotlight menulet).
You can tab to the 'Keyboard' / 'Keyboard Shortcuts' tab bar, then arrow over to 'Keyboard Shortcuts', then tab to the 'Full Keyboard Access' radio buttons. Sadly, these last few steps require that Full Keyboard Access already be turned on! The text below the radio buttons says that you can toggle the setting with CTRL-F7, but that doesn't work for me; I'm not sure if some other keybinding is interfering.
(as root)
# ps axo ucomm | grep -q SecurityAgent && osascript -e 'tell application "SecurityAgent" to activate'Command + first letter button title
will activate that option, with a few caveats - Command-C won't work as Cancel, but Command-. will.
Or just hit Control-F7.