[no squash] Avoid use of select(2)

[sfan5]

select() is a fundamentally broken API that shouldn't be used anymore. So this is what we'll do.

This raises the minimum curl version to at least 7.28.0. It was released sometime in 2012.

To do

This PR is Ready for Review.

How to test

1. try single player
2. try mainmenu
3. try this nearly pointless mod that performs HTTP requests:

local http_api = core.request_http_api()
assert(http_api)

local testn = 0
local function dotest()
	if testn >= 6 then
		print("done")
		return
	end
	testn = testn + 1
	local my_n = testn
	print("begin " .. my_n)
	http_api.fetch({
		url = "https://httpbin.org/delay/2",
	}, function(res)
		print("end " .. my_n .. " " .. tostring(res.succeeded))
	end)
	core.after(1 + testn/4, dotest)
end
core.after(3, dotest)

4. ensure it doesn't hang at shutdown (any more than before)

[grorp]

ensure it doesn't hang at shutdown

This doesn't happen to have something to do with #12815, does it? I did a short test and Minetest still hangs if I try to close it while downloading a package from ContentDB.

[sfan5]

This doesn't happen to have something to do with #12815, does it? I did a short test and Minetest still hangs if I try to close it while downloading a package from ContentDB.

Not related, no.
We had #12983 for that but IMO it would be better for the mainmenu to use the async httpfetch API. Then this problem would also not exist.

[SmallJoker]

Media loading still works. Looks good.