Changed layout of $ZEROINSTALL_PORTABLE_BASE

I've just committed a change to the layout of the directories when
$ZEROINSTALL_PORTABLE_BASE is used:

- Removed "0install.net" from the start of the paths.

- Non-0install things now always get looked up in $XDG_* and aren't
affected by the portable base (I think the only thing we use this for
is $XDG_DATA_DIRS/applications anyway).

Hopefully this doesn't affect anyone. The change it to make it match
the Windows version, which I think was the only user:

  https://github.com/0install/0install/issues/50


-- 
Dr Thomas Leonard        http://roscidus.com/blog/
GPG: DA98 25AE CAD0 8975 7CDA  BD8E 0713 3F96 CA74 D8BA

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