[NOSQUASH] Fix some warnings

[Desour]

• An intentional self-move in the irr_ptr unittests.
  (Note: If not defined, macros, such as __GNUC__, are replaced 0, so this is fine.)
• The minimum curl version is now specified as 7.68.0. This is the version on ubuntu 20.04.
  The actual minimum is probably 7.56.0, but I'm not sure if the other parts of the code use newer functions.
  FYI, ubuntu 18.04 seems to have curl 7.58.0.
  The minimum curl version is now specified as 7.56.0.
• The invlist warnings were introduced recently.
  (I'm assuming invlist size does not use the full range of values in u32. We should really start using one datatype for invlist indices everywhere at some point...)

To do

This PR is a Ready for Review.

How to test

• Build.
• Example to try the multipart http stuff:

local http = assert(minetest.request_http_api())

minetest.after(3, function()
	local req = {
		url = "https://httpbin.org/post",
		method = "POST",
		data = {foo = "bar", baz = "buzz", bla = "bli"},
		user_agent = " ",
		multipart = true,
	}
	http.fetch(req, function(res)
		minetest.log(dump(res))
	end)
	minetest.log("sent http request")
end)

[JosiahWI]

I had noticed the warnings and they were distracting me. I like the prospect of having these fixed.

[lhofhansl]

What type does getSize return? We do not need 32 bits here, should change the type of i instead?

[Desour]

getSize returns an u32.
InventoryList (see inventory.h) seems to use u32 everywhere. But there's also for example MoveAction (see inventrymanager.h), which uses s16.
It would probably be good to use s32 everywhere (after all u16 could get too small for things like creative inventories). But doing that change would require more work.

[sfan5]

LGTM