Still cope compared to the MP addfile codepath, but not the n^2 time cope of before.

Tested with a full 255 WAD server, it's a LITTLE silly but I think it's inexplicably safe to ship!?

- Make all the I_Errors return false and print to the console instead.
- New prints for missing files if you can't fit it all into one packet.
- Make the startmessage warning less specific and direct you to the logfile, to accomodate all the different ways legacy downloads can fail.

- Catch buffer overrun opportunities and fail early.
- Add #define MORELEGACYDOWNLOADER for the equivalent of MOREFILENEEDED, but disabled for now because honestly we really shouldn't be encouraging people to use this thing by making it support 255 WADs at once, but also because that'd be MISERABLE to test
- Add a menu report for when legacy downloader attempts fail

It's the collective opinion of Kart Krew's coders that sticking to C90 spec isn't healthy... but while the compiler still warns for it in the v1 buildflags, avoid undesired reports.

# Conflicts:
#	src/p_tick.c

Fixes crashes for newly-written demos that have files with absurdly long names attached

# Conflicts:
#	src/p_tick.c

Another spot that needs guarding

See merge request KartKrew/Kart-Public!320

Ensure skin index read from replay file is within numskins before drawing face

See merge request KartKrew/Kart-Public!319

Fixes desyncs with rotating polyobjs

If you're looking to this commit to figure out what a hypothetical 1.7 should change, also increment MODVERSION - in this commit's case it was already incremented by another branch.

Adjusts pview_old for relative teleport to attempt a contigious motion

Make all the join/leave sound effects have a `singularity` of `true` so that they don't stack super loud during major net interruption or multiple splitscreen player joins.

Instead of not running for server or admin, run for ALL players, but only avoid *kicking* local or admin players.
Fixes a bug where having a non-interacting splitscreen player could take down your server.