Eliminate the use of explicit file separators

[lhstrh]

There are some occurrences of explicit file separators (/) that may lead to problems on Windows. No such problems have shown up in regression testing on Windows, however, so this doesn't deserve high priority, but it would be a good idea to audit the repo for this. E.g.:

The slash here doesn't look platform independent, but the code that this replaces also featured slashes...

Originally posted by @lhstrh in #1914 (comment)