Pocketmine-mp: Change unknown byte in entity links

[thebigsmileXD]

Issue description

The software still names the 4th parameter in SetEntityLinkPacket as unknown, though it's usage is already known for a while now: which seat the entity will be linked to. _EDIT: see @dktapps reply below, was mistaken there_

Entities that can be ridden can have multiple seats, example: boats. Boats have a front and a back seat. The byte decides where you sit at, and combined with DATA_CONTROLLING_RIDER_SEAT_NUMBER it can be used to make passengers + "drivers"

• Expected result: What were you expecting to happen?
• Actual result: What actually happened?

Steps to reproduce the issue

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2. ...

OS and versions

• PocketMine-MP: *
• PHP: 7.2
• Server OS: *

Plugins

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Crashdump, backtrace or other files

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• ...

[dktapps]

Actually, it is a boolean which determines whether the rider should be directly removed or flagged to be removed later.