Final code fixes and minor cleanup

• Fixed a couple crashes on insert-typing a char at index 0, and pasting at index 0. The former was my fault when switching INT32 to unsigned.
• Activate Kart splitscreen chat code behind #define NETSPLITSCREEN
• Disable chat by adding splitscreen check to OLDCHAT
• Draw up and down scroll arrows the 2.2 way, by string
• Cleanup commit to adjust comment whitespace and rename stop_spamming_you_cunt (yeah fuckaj can stay but I draw the line at cunt, sorry :X )

Pending whether I can actually draw the arrows correctly; hold tight while I figure out why SLADE's not firing up... works fine, fire away

src/d_net.h

@@ -22,6 +22,7 @@
 #define MAXNETNODES 32
 #define BROADCASTADDR MAXNETNODES
 #define MAXSPLITSCREENPLAYERS 2 // Max number of players on a single computer
+//#define NETSPLITSCREEN // Kart's splitscreen netgame feature
 
 #define STATLENGTH (TICRATE*2)