Reorder client initialization

[SmallJoker]

Before

minetest.get_connected_players occasionally returns object references of yet-not-joined players, for example in minetest.register_globalstep callbacks. This is a problem because minetest.register_on_joinplayer was not called yet.
Mods rely on joinplayer to initialize their API entries, which causes unpredictable and unreliable code paths.

After

minetest.get_connected_players includes the player reference only after (and including) minetest.register_on_newplayer, followed by minetest.register_on_joinplayer.

Fixes the root issue of minetest-mods/playeranim#3

To do

This PR is Ready for Review.

How to test

1. https://github.com/minetest-mods/playeranim commit 9dab03f
2. This code:

-- Ensure the callbacks are run
minetest.register_on_newplayer(function(p)
	print("newplayer", p:get_player_name())
end)
minetest.register_on_joinplayer(function(p)
	print("joinplayer", p:get_player_name())
end)

3. Any other mod you'd like to test.
4. Host a server and attempt to join with a same-named player

[sfan5]

This is not the same issue as #12073, is it?

[SmallJoker]

Actually yes, it seems to be the same issue.

[sfan5]

The other report came from a faulty third party client causing the engine to become confused. If this one is reproducible in the normal client then it's not the same issue.

[SmallJoker]

It is the same as in #12073 (comment) but reproducible at a chance with the official Minetest client.

[sfan5]

FYI I haven't reviewed this yet because I'm unsure whether this is a good idea (especially the setNewPlayer feels wrong) but haven't had time to investigate alternatives.

[Zughy]

@SmallJoker rebase needed

[Zughy]

IRC discussion: https://irc.minetest.net/minetest-dev/2023-03-05#i_6060589 (in short, it'll wait a bit)

[SmallJoker]

For some reason I can no longer reproduce the bug in master, even though the relevant code does not seem to have changed in any meaningful way. This PR no longer seems to be necessary.