[UserMetaData] Untangle Windows, Linux/BSD/Mac and stub code.

Summary:
The Windows code only shares the function signature with the other
variants, while the actual code is vastly different. One example are
path and attribute names, which are UTF-16 on Windows and UTF-8 otherwise,
but QFile::encodeName() is also called on Windows.

This fixes also two small bugs on Windows, one missing CloseHandle in
case the attribute has a size of 0, and a wrong return value in case
writing the ADS fails.

Test Plan:
Linux: ctest
Windows/Mac/*BSD: untested

Reviewers: Baloo, Frameworks, Windows, FreeBSD, ngraham, astippich

Reviewed By: astippich

Subscribers: adridg, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20133