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/qcommon/md5.c

@@ -78,7 +78,7 @@ static void MD5Init(struct MD5Context *ctx)
 static void MD5Transform(uint32_t buf[4],
 	uint32_t const in[16])
 {
-    register uint32_t a, b, c, d;
+    uint32_t a, b, c, d;
 
     a = buf[0];
     b = buf[1];