Win32: Decouple the config files and memstick directory from the binary, unless portable.txt exists, or we can't find the user's "home" directory(e.g. My Documents on XP or Documents on Vista/7/8).
Update GetSysDirectories to use g_Config.memCardDirectory.
Add blank portable.txt to keep current behaviour for developers.
If we're in Program Files or can't write to the directory for some reason, disable portable mode.
Remove stupid attempt at detecting a read-only directory, and instead use the proper Vista/7/8 XML element. PPSSPP should run as "asInvoker" so that the Virtual Store isn't enabled for it. This way, if we fail to write to PPSSPP's own directory, we'll branch out to the My Documents/Documents folder.