Rewrite build_spool_path to be mostly sane^W^Wnot utterly insane

* Now actually generates paths with single slashes.
* Uses fewer magic numbers for string buffer lengths.
* Uses stpncpy to basically Do What I Mean.
* Have you ever even heard of stp(n)cpy? Yeah me neither.
* Such comment. Very understanding. Wow.
Merge branch 'master' of barneydesmond.github.com:anchor/libmarquise