Use SDL version of executable icon at runtime on macOS

See merge request !321

Fix game trying to load nothing when music and sound files are not present.

See merge request !322

So that it actually falls back to direct downloading

In cases of where the file mismatches from what the server expects, it will fall back to direct downloading of the file.

- Fixed off by one in Got_RequestFilePak that could cause correct, maximised-space-usage packets to be rejected
- More verbose printing for aborting send files
- More verbose printing for client request files (behind a define for troubleshooting)

Skips resync, clearticcmd, and sendtics, since all they're doing with no nodes in game is writing the same value to the same address again and again

There's enough confidence among krew that there's zero apparent downside to this, but made sure to do it as a seperate commit if we have to hit the emergency switch and revert

Same rules as thunder shield - the SPBis coming out again ASAP.

- If no clients at server start or after 10 seconds of GS_LEVEL, and no Netxcmd waiting to be digested, halt all SV_MakeTic.
- Currently #define'd out, but if we don't get to test it before 1.6 release, I fully encourage community build developers to enable this codepath and trial it on their servers.
- It's absolutely netsafe to only have enabled on the host's end, the only risk is that a dedicated server might not re-awaken when presented with certain stimuli.

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