This allows mods to perform both asynchronous and synchronous HTTP
requests. Mods are only granted access to HTTP APIs if either mod
security is disabled or if they are whitelisted in any of the
the secure.http_mods and secure.trusted_mods settings.

Adds httpfetch_caller_alloc_secure to generate random, non-predictable
caller IDs so that lua mods cannot spy on each others HTTP queries.

Instead, perform a POST request with post_data.

Closes #3748

Fixes #3378
Closes #3751

Also, downgrade the error to a warning.

Previously you could steal a secure environment from a trusted mod by wrapping
request_insecure_environment with some code like this:

local rie_cp = minetest.request_insecure_environment
local stolen_ie
function minetest.request_insecure_environment()
	local ie = rie_cp()
	stolen_ie = stolen_ie or ie
	return ie
end

'cam_pos_nodes -/+ box_nodes_d' can exceed the range of v3s16
when a player is near the world edge using a large view range
This previously caused world to disappear
Create new function getBlocksInViewRange() called from
updateDrawList() and renderMap()
Correct code style throughout updateDrawList() and renderMap()

Commit

6a1670db "Migrate to STL containers/algorithms."

has replaced core::map<T, bool> with std::set<T>, but didn't update
the accompanying comment saying "value is dummy". This happened at
two places.

Fixes #3701.

Thanks @ExcaliburZero for noticing. Fixes #3698.

* Sky: rename Box => m_box and inline getBoundingBox
* Uniformize aabbox3d<f32> to aabb3f

This makes the functions a bit faster since they don't
have to recreate the tables every invocation, and makes
the code more readable.

Also, document `wallmounted_to_dir`.

The function was implemented but not documented in `lua_api.txt`.

Introduce a new contentfeatures version (8). When clients
connect using v27 protocol version, they can assume that
the tiledef.backface_culling is trustable, but if clients
connect to servers providing contentfeatures version 7,
then the v27 clients know that backface culling settings
provided by the server in tiledefs are bogus for mesh,
plantlike, firelike or liquid drawtype nodes.

thanks to hmmmm, est31, nerzhul.

Tested on new client - new server, new client - old server
old client - new server.

Also use string::empty method, it is better than comparing with empty strings.

All places that lock this mutex are only called by the main thread:
ClientMap::updateCamera(), ClientMap::updateDrawList(), ClientMap::renderMap(), ClientMap::renderPostFx().

To avoid spawn search failing in new specialised mapgens
Increase spawn search range to 4000 nodes
Add getSpawnLevelAtPoint() functions to EmergeManager, class Mapgen
and all mapgens
Remove getGroundLevelAtPoint() functions from all mapgens except mgv6
(possibly to be re-added later in the correct form to return actual
ground level)
Make mgvalleys flag names consistent with other mapgens
Remove now unused 'vertical spawn range' setting