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.

sshbn.c

@@ -1404,8 +1404,9 @@ void decbn(Bignum bn)
     bn[i]--;
 }
 
-Bignum bignum_from_bytes(const unsigned char *data, int nbytes)
+Bignum bignum_from_bytes(const void *vdata, int nbytes)
 {
+    const unsigned char *data = (const unsigned char *)vdata;
     Bignum result;
     int w, i;
 
@@ -1426,8 +1427,9 @@ Bignum bignum_from_bytes(const unsigned char *data, int nbytes)
     return result;
 }
 
-Bignum bignum_from_bytes_le(const unsigned char *data, int nbytes)
+Bignum bignum_from_bytes_le(const void *vdata, int nbytes)
 {
+    const unsigned char *data = (const unsigned char *)vdata;
     Bignum result;
     int w, i;