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.

windows/window.c

@@ -3083,7 +3083,7 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
 		     */
 		    term_seen_key_event(term);
 		    if (ldisc)
-			ldisc_send(ldisc, (char *)buf, len, 1);
+			ldisc_send(ldisc, buf, len, 1);
 		    show_mouseptr(0);
 		}
 	    }
@@ -5921,12 +5921,12 @@ void frontend_keypress(void *handle)
     return;
 }
 
-int from_backend(void *frontend, int is_stderr, const char *data, int len)
+int from_backend(void *frontend, int is_stderr, const void *data, int len)
 {
     return term_data(term, is_stderr, data, len);
 }
 
-int from_backend_untrusted(void *frontend, const char *data, int len)
+int from_backend_untrusted(void *frontend, const void *data, int len)
 {
     return term_data_untrusted(term, data, len);
 }

windows/winhsock.c

@@ -137,14 +137,14 @@ static void sk_handle_close(Socket s)
     sfree(ps);
 }
 
-static int sk_handle_write(Socket s, const char *data, int len)
+static int sk_handle_write(Socket s, const void *data, int len)
 {
     Handle_Socket ps = (Handle_Socket) s;
 
     return handle_write(ps->send_h, data, len);
 }
 
-static int sk_handle_write_oob(Socket s, const char *data, int len)
+static int sk_handle_write_oob(Socket s, const void *data, int len)
 {
     /*
      * oob data is treated as inband; nasty, but nothing really

windows/winnet.c

@@ -931,8 +931,8 @@ static void sk_tcp_flush(Socket s)
 }
 
 static void sk_tcp_close(Socket s);
-static int sk_tcp_write(Socket s, const char *data, int len);
-static int sk_tcp_write_oob(Socket s, const char *data, int len);
+static int sk_tcp_write(Socket s, const void *data, int len);
+static int sk_tcp_write_oob(Socket s, const void *data, int len);
 static void sk_tcp_write_eof(Socket s);
 static void sk_tcp_set_frozen(Socket s, int is_frozen);
 static const char *sk_tcp_socket_error(Socket s);
@@ -1568,7 +1568,7 @@ void try_send(Actual_Socket s)
     }
 }
 
-static int sk_tcp_write(Socket sock, const char *buf, int len)
+static int sk_tcp_write(Socket sock, const void *buf, int len)
 {
     Actual_Socket s = (Actual_Socket) sock;
 
@@ -1588,7 +1588,7 @@ static int sk_tcp_write(Socket sock, const char *buf, int len)
     return bufchain_size(&s->output_data);
 }
 
-static int sk_tcp_write_oob(Socket sock, const char *buf, int len)
+static int sk_tcp_write_oob(Socket sock, const void *buf, int len)
 {
     Actual_Socket s = (Actual_Socket) sock;
 

windows/winplink.c

@@ -104,7 +104,7 @@ void frontend_echoedit_update(void *frontend, int echo, int edit)
 char *get_ttymode(void *frontend, const char *mode) { return NULL; }
 
 int from_backend(void *frontend_handle, int is_stderr,
-		 const char *data, int len)
+		 const void *data, int len)
 {
     if (is_stderr) {
 	handle_write(stderr_handle, data, len);
@@ -115,7 +115,7 @@ int from_backend(void *frontend_handle, int is_stderr,
     return handle_backlog(stdout_handle) + handle_backlog(stderr_handle);
 }
 
-int from_backend_untrusted(void *frontend_handle, const char *data, int len)
+int from_backend_untrusted(void *frontend_handle, const void *data, int len)
 {
     /*
      * No "untrusted" output should get here (the way the code is