Make lots of generic data parameters into 'void *'.

This is a cleanup I started to notice a need for during the BinarySink
work. It removes a lot of faffing about casting things to char * or
unsigned char * so that some API will accept them, even though lots of
such APIs really take a plain 'block of raw binary data' argument and
don't care what C thinks the signedness of that data might be - they
may well reinterpret it back and forth internally.

So I've tried to arrange for all the function call APIs that ought to
have a void * (or const void *) to have one, and those that need to do
pointer arithmetic on the parameter internally can cast it back at the
top of the function. That saves endless ad-hoc casts at the call
sites.

cproxy.c

@@ -132,7 +132,7 @@ int proxy_socks5_handlechap (Proxy_Socket p)
 		hmacmd5_chap(data, p->chap_current_datalen,
 			     conf_get_str(p->conf, CONF_proxy_password),
 			     &outbuf[4]);
-		sk_write(p->sub_socket, (char *)outbuf, 20);
+		sk_write(p->sub_socket, outbuf, 20);
 	      break;
 	      case 0x11:
 	        /* Chose a protocol */