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.

pageant.c

@@ -403,7 +403,7 @@ void pageant_handle_msg(BinarySink *bs,
             put_byte(bs, SSH2_AGENT_SIGN_RESPONSE);
 
             signature = strbuf_new();
-            key->alg->sign(key->data, (const char *)data, datalen,
+            key->alg->sign(key->data, data, datalen,
                            BinarySink_UPCAST(signature));
             put_stringsb(bs, signature);