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.

sftp.c

@@ -993,7 +993,7 @@ struct fxp_names *fxp_readdir_recv(struct sftp_packet *pktin,
  * Write to a file. Returns 0 on error, 1 on OK.
  */
 struct sftp_request *fxp_write_send(struct fxp_handle *handle,
-				    char *buffer, uint64 offset, int len)
+				    void *buffer, uint64 offset, int len)
 {
     struct sftp_request *req = sftp_alloc_request();
     struct sftp_packet *pktout;