Exit your server desync fix

This branch fixes this bug: https://mb.srb2.org/showthread.php?t=41811

Here's an explanation of how this bug is actually happening:

• Pressing Enter with "Start" selected calls D_MapChange to make a map change command. (Note that part of it is actually delayed because it's called from a menu, but this isn't relevant to the bug)
• D_MapChange itself calls SV_StartServer to start up the server, if you're supposed to be the server player.
• Pressing Esc while starting up a server like shown in the gif causes the server to be cancelled, and go back to the title screen.
• HOWEVER, since we're still in D_MapChange, the map change command is made anyway.
• Since we're not in a netgame as expected, no random seed is added, and the map change command has a total size of 9 bytes (1 for the id + 8 for the actual data). This is then stored in the net command buffer. The random seed would add another 4 bytes if it WAS added, note.
• Next time you start a server, and let it work as normal, two more net commands are added: an "add player" command (size 3), and a map change command (size 13, including random seed this time). Your net command buffer now has a total size of 25 (9 + 3 + 13).
• This net command buffer is then sent as a PT_TEXTCMD packet to yourself.
• SRB2 recieves the PT_TEXTCMD you sent to yourself, and some time later, tries to run the net commands stored in it.
• The first map command is read as if it's size 13 rather than size 9 (it isn't), mistakeningly thinking a 4-byte random seed is stored (since you ARE in a netgame now). This means your buffer reads what it thinks is the next net command starting from position [14] in the buffer.
• Bad luck, it gets an id of 0, which doesn't correspond with any net command!
• This message is then printed in the console:

WARNING: Got unknown net command [14]=0 (max 25)

• The game subsequently kicks you out the server with a "desynch" message.

tl;dr The game doesn't correctly cancel the map change when you cancel the server with Esc like that. This is not a good thing.