Avoid 'register' declarations

gcc 6 with -Wall -Wextra warns:

code/botlib/l_precomp.c: In function ‘PC_NameHash’:
code/botlib/l_precomp.c:551:2: warning: ‘register’ is not at beginning of declaration [-Wold-style-declaration]
  int register hash, i;
  ^~~

Modern compilers either ignore the register storage class when
generating code, or generate better code without it, so just remove
most of them.

The remaining uses are in third-party bundled libraries (libjpeg, zlib),
and in a PowerPC-specific inline function consisting of inline
assembler (because I'm not 100% confident that it doesn't have
some practical use there).

code/game/bg_lib.c

@@ -53,10 +53,10 @@ static void	 swapfunc(char *, char *, int, int);
  */
 #define swapcode(TYPE, parmi, parmj, n) { 		\
 	long i = (n) / sizeof (TYPE); 			\
-	register TYPE *pi = (TYPE *) (parmi); 		\
-	register TYPE *pj = (TYPE *) (parmj); 		\
+	TYPE *pi = (TYPE *) (parmi); 			\
+	TYPE *pj = (TYPE *) (parmj); 			\
 	do { 						\
-		register TYPE	t = *pi;		\
+		TYPE	t = *pi;			\
 		*pi++ = *pj;				\
 		*pj++ = t;				\
         } while (--i > 0);				\