Implement "synology" compat option for Synology NAS

[berni2288]

By default, the Synology "btrfs send" command will send a lot of ACLs and maybe other information, a standard btrfs can't deal with. I've discovered that it's possible to turn this behavior off with --without-syno-features and let it behave like a normal btrfs send, allowing btrbk to send Snapshots to a non Synology btrfs.

Maybe #383 already tried to address this issue, but for me the btrbk run still failed. With this change here, all receive errors are gone.

I've also added an example in the config how to archive all Synology subfolders at once.

[berni2288]

@digint This is an optional feature which is not enabled by default. Do you need any improvements for this MR before it can be merged? Thanks!