Re: [WIP] grub-btrfs: update to 4.8.1

[FollieHiyuki <FollieHiyuki@users.noreply.github.com>]

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New comment by FollieHiyuki on void-packages repository

https://github.com/void-linux/void-packages/pull/28288#issuecomment-772252089

Comment:
Thanks @ahesford for all the feedbacks.
The only features of btrfs I use are compression and snapshoting, so I doubt I know any better :)
> Omit the -v here and add some logic that will recognize a variable like VERBOSE that can be set in the conf file to govern the addition of -v. This could be a catch-all OPTS variable like other services uses, but there probably isn't much value in allowing other flags, since you want tighter control on the path and mode anyway.

I keep the OPTS variable. There could be a use case for it, for example `-r` when the user manages snapshots inside multiple sub directories. Adding `-d` would be a sanity check too.
> Is mode 768 sufficient? If snapshots can be renamed, you might also need bits for IN_MOVE or the IN_MOVED_{TO,FROM}. You should do some experiments to see what kind of events a snapshot create/rename/destroy trigger.

I added IN_MOVE bit.
AFAIK btrfs snapshots are subvolumes, and they can be treated as normal directories (delete wirh `rm -r`, rename wirh `mv`) so I guess it emits events as normal directories in the file system. Apparently the service at least works on my system with all 3 events.
> You should probably verify that SNAPSHOTS_PATH is non-empty and either fail or substitute a default when it is.

Since `wendy` will do nothing if the directory doesn't exist, I added the check for that case. Having to check for SNAPSHOTS_PATH value as well will be complicating, so I keep a default value in the `conf` file