nixos-manual: Simplify stripping prefixes

Let's use a simple (unflipped) fold and break out the actual core
stripPrefix function from stripAnyPrefixes (I personally love
point-less^H^H^H^Hfree style but if I'd be anal I'd even go further and
factor away the "fn:").

Also, let's use path as a better name for "fn" (filename), because
that's what it is and also cannot be confused with "fn" meaning
"function".

We now toString all of the prefixes, so there shouldn't be any need to
implicily toString the extraSources anymore.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>