Update getHomeDir() to use "os/user"

os.Getenv("HOME") doesn't work properly on Windows (and maybe other systems?) and the package "os/user" gives us a convenient way to find the home directory for every OS.
Set the right config and home dir for each OS

Use %AppData%\syncthing for the config files and %USERPROFILE%\Sync as sync folder for Windows.
Delete cfgFile before renaming it on Windows
Fix divided by zero when the sync folder is empty (tot = 0)
Merge branch 'patch-1'