"Asynchronous" file queue calls findmultiplefiles on another thread, then reads the file on the main thread

This replaces the exec command's threading from before.
(See d0e7ba30.) The order of exec commands
alone is guranteed. And it removes the need to put mutexes on the command
buffer. I plan to use this system for addfile too...

This speeds up file searching for base resources and pwads by using one
directory traversal. Previously findfile could be called for each wad. And
although subsequent traversals after the first are faster, it's still a waste.

Connection screen improvements

See merge request KartKrew/Kart-Public!187

2.2's random client port

See merge request KartKrew/Kart-Public!201

Instantaneous lookback camera (v2 backport)

See merge request KartKrew/Kart-Public!203

GME cleanup and fixes

See merge request KartKrew/Kart-Public!202

If we initialize to I_GetTime() - NEWTICRATE*3 it never underflows and everything is good

Just some stuff backported from SRB2 2.2, well mostly

get_zlib_error function to reduce code duplication

Return false if it fails to load the VGZ lump

Reduce redundant calls to gme_set_equalizer

Fix sometimes loading a VGZ crashing the game

If you ever need to, you can change the client port number.

(cherry picked from commit 843d9b9f)

Using strcpy is stupid because we don't know how long the argument would be.
There's no need for a buffer anyway.

(cherry picked from commit 4e321012894b2aa87e312597f19be26b30545a8b)

(cherry picked from commit f584b61c)

This fixes '-connect'. NEWTICRATE*5 is subtracted from asksent after
CL_LOADFILES. If this happens too early, an underflow will occur.

Fix NOCURL compiling

See merge request KartKrew/Kart-Public!197

Let servers send httpsource without curl

See merge request KartKrew/Kart-Public!196

Lua Input/Output library

See merge request KartKrew/Kart-Public!200

[OSX] Fix "SIGILL: illegal instruction" on macOS Catalina

See merge request KartKrew/Kart-Public!194

also remove some extra tab characters

mainwads+1 to not use the first mainwad

Downloads must be confirmed before proceeding
Allows downloading mods for full servers
Client will wait for a slot to open on a full server to join
This has a 5 minute timeout

Resolution in particular relies on this to be done in order...