syncobj: check that all path components are not files when creating directories

When trying to download a file check if any of its directory fragments that would be recursively created exist as a file.

This fixes a bug where we only checked if the directory key was a file, but this will not catch cases like this:

downloading directory foo/bar/baz/

We needed to check if foo, foo/bar for foo/bar/baz are files in the
local filesystem.  We were only checking foo/bar/baz

Resolves #40 (closed)