clean up file drivers: drivers now always receive paths with trailing slashes;

this makes handling the relative "root" directory not a special case (".").
mountpoints now always begin with a slash.  before, "/foo" and "foo" were
in two separate namespaces, which was weird and confusing; the two now
mean the same thing.  there are no more special "default mountpoints"; just
mount to "/".  "@path" mounts are now "/@path", and they do show up in
GetDirListing("/") (but, as before, opening "/@foo/bar" will never create
a "@foo/bar" path in a non-"@foo" mountpoint)

stepmania/src/RageUtil_BackgroundLoader.cpp

@@ -83,7 +83,7 @@ CString BackgroundLoader::GetRequest()
 
 CString BackgroundLoader::GetCachePath( CString sPath ) const
 {
-	return m_sCachePathPrefix + "/" + sPath;
+	return m_sCachePathPrefix + sPath;
 }
 
 void BackgroundLoader::LoadThread()