synced_folders/rsync: enable override of default chmod flag

The default `--chmod` flag on rsync can be overridden on Windows now. 

If no custom `--chmod` flag is specified and `--archive` is used, the `--no-perms` flag is added to the arguments, because `--archive` automatically adds the `--perms` flag which in turn would ignore the `umask` on the guest.

Quote from `man rsync`:

> To give new files the destination-default permissions (while leaving existing files unchanged), make sure that the --perms option is off and use --chmod=ugo=rwX (which ensures that all non-masked bits get enabled).  

I think, this new default behavior is feasible for most Windows users. Do you agree?