Add support for a get_keys() equivalent for lower than 5.7 Minetest versions

[Athozus]

Fix #152

@MoNTE48 can you confirm me that it works ?

Notes :

• It checks if get_keys() exists
• Then, if not, it iterates through existing players (but consume more)
• I've also reorganized into more functions to not have too much duplicates

[MoNTE48]

@luk3yx

[Athozus]

Btw, normally until the repair hasn't finished, it is still noted as v3, but be sure that it is in order to run the repair. And do a backup, as I don't really want to break my Minetest by regressing version 😁 .

[luk3yx]

There's another call to mail.storage:get_keys() in is_uuid_existing without a check that causes a crash.

[Athozus]

Is it okay now @luk3yx ?

[luk3yx]

It works now, thanks!