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.

proxy.c

@@ -96,7 +96,7 @@ static void sk_proxy_close (Socket s)
     sfree(ps);
 }
 
-static int sk_proxy_write (Socket s, const char *data, int len)
+static int sk_proxy_write (Socket s, const void *data, int len)
 {
     Proxy_Socket ps = (Proxy_Socket) s;
 
@@ -107,7 +107,7 @@ static int sk_proxy_write (Socket s, const char *data, int len)
     return sk_write(ps->sub_socket, data, len);
 }
 
-static int sk_proxy_write_oob (Socket s, const char *data, int len)
+static int sk_proxy_write_oob (Socket s, const void *data, int len)
 {
     Proxy_Socket ps = (Proxy_Socket) s;