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/botlib/l_precomp.c

@@ -548,7 +548,7 @@ void PC_PrintDefineHashTable(define_t **definehash)
 
 int PC_NameHash(char *name)
 {
-	int register hash, i;
+	int hash, i;
 
 	hash = 0;
 	for (i = 0; name[i] != '\0'; i++)