Broken links on Windows

[Kwpolska]

• On AppVeyor:

[2015-06-04T08:34:18Z] WARNING: check: Broken link in output\listings\index.html: https://example.com/foo/listings/index.html.html

• On my computer: invalid, symlinks issue

[2015-06-04T08:56:23Z] WARNING: check: Broken link in output\index.html: stories/handbook.html
[2015-06-04T08:56:23Z] WARNING: check: Broken link in output\posts\welcome-to-nikola.html: ../stories/handbook.html

[Kwpolska]

<link rel="canonical" href="https://example.com/foo/listings/index.html.html">

This was caused by checking for / instead of os.sep in listings.py, even though we explicitly convert everything to os.sep a few lines above.

Along the way, I found another bug: os.split was used to generate the path parts, even though it makes only one split (and we need all). #1785; cc @felixfontein