Fix the replay menu for people with non-latin characters in their username.

Use OsPath instead of CStr and CStrW where possible,
wstring_from_utf8(OsPath.string8()) to pass printable strings to the JSAPI,
OsString when opening a filestream and
off_t instead of int for filesizes.

Fixes #4320
Differential Revision: https://code.wildfiregames.com/D518
Reviewed By: Imarok
Tested By: Imarok on Windows, wraitii on OSX
Special thanks to Philip for advice and the lib/path.h fix in rP19823.