Handle valid non-separator backslash characters in filepaths passed to expand_path

[phargogh]

In expand_path (#1477), we are now checking for the existence of \ characters in filepaths provided to the function and, if we are on a linux/mac system, assuming that this is a windows path separator and converting \ to /.

Unfortunately, \ is a valid filepath character on linux/mac, so this is not a truly safe assumption to make. It would be safer to, when passed a filepath containing \, handle the possibility that one or more of the \ characters are valid path characters and return the correct, sanitized path.