When a user sets 'background: none' it really means "Do not let themes touch the
background I have already set with fbsetbg" -> It does still actually call
fbsetbeg to re-assert the proper background.
This new value 'unset' is for users who will use an alternate method to set
their background (root-tail, xscreensaver, or whatever). It instructs fluxbox
to *never* run fbsetbg.
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