Feature: Allow mergerfs.balance to exclude target path/disk

[undaunt]

Our prior conversations are here and here

Allow mergerfs.balance to exclude a disk or path from being a target of data moved during the balance process

Use case:

disk1 (3%) - VHS content
disk2 (90%) - VHS content
disk3 (90%) - VHS content
disk4 (1%) - Docker volumes, databases, etc.

Disks 1, 2, and 3 contain /VHS root folder, Disk 4 does not. Mergerfs is running with a path preserving policy, such as empfs.

If running mergerfs.balance, I'd like to be able to exclude any data from being moved to disk4. Since balance is not path preserving, I can't currently prevent it from creating the VHS folder on disk4, but if I can exclude disk4 from being a recipient of any transferred data, this is possible, regardless of it not respecting path preservation.