Implement search tab and version picker
Add configurable font shadow.
Add 'on_prejoinplayer' callback
Make line_of_sight return blocking node position
Fix a bug in GenericCAO GENERIC_CMD_UPDATE_POSITION parsing.
Implement httpfetch module and initialize it from main()

Add curl_parallel_limit setting that will replace media_fetch_threads in
a later commit.

Fix a typo in MutexedQueue::pop_back() that made it impossible to compile
code that used this function. (Noticed this while implementing httpfetch.)
Rewrite client media download and support hash-based remote download

Move most of the media-related code in client.cpp into a new class
ClientMediaDownloader (clientmedia.cpp, clientmedia.h). Among other
things, this class does the following things:

- Download [remote_server][sha1] instead of [remote_server][name]. This
is to support servers that provide the same file name with different
contents.
- Initially fetch [remote_server]index.mth. This file should follow the
Minetest Hashset format (currently version 1) and contain a list of SHA1
hashes that exist on the server.
- The list of needed SHA1s is uploaded (via HTTP POST) when index.mth is
requested, so servers can optionally narrow down the list to the needs
of the client.
- If index.mth is missing (HTTP response code 404), we enter compat mode,
fetching [remote_server][name] as before this commit.
- remote_server can now contain multiple servers, separated by commas.
The downloader code attempts to split requests between the different
servers, as permitted by each server's index.mth. If one server claims
to have a file but actually doesn't (or something fails), we ask a
different server that also claims to have it.
- As before, when none of the remote servers provide a particular
file, we download it via the conventional method, i.e. using
the minetest protocol: TOSERVER_REQUEST_MEDIA / TOCLIENT_MEDIA.
- Bugfix: Every downloaded file's SHA1 is now verified against the SHA1
announced by the minetest server (before loading it and inserting it
into the file cache).
- Bugfix: Only send TOSERVER_RECEIVED_MEDIA when we actually have all
media. This should fix #863.
Implement urlencode and urldecode
Use httpfetch_async in serverlist announce code
Replace any direct curl usage by httpfetch
Don't call curl_global_init twice
Remove support for optdepends.txt
Add map feature generation notify Lua API
Handle Lua errors in on_generate callbacks instead of throwing SIGABRT
Update mapgen params in ServerMap after Mapgen init
Add alpha setting to font shadow
Replace SimpleThread by JThread now implementing same features
Directional fog + horizon colors, based on sun & moon positions at sunrise / sunset
Add setting to disable direction dependent fog and sky colors
Remove an unused function and include from client.cpp