Fix FTBFS when using -pedantic compiler flag.

I am attaching an incremental git-am ready patch to go on top your Git HEAD,
to fix all sorts of issues and make this conforming C99 with default
options set,
and fix another load of warnings you receive when setting the compiler
to pick the nits,
-pedantic-errors -std=c99 (or c11, c18, c2x).
It changes many void * to uint8_t * to make the "increment by bytes"
explicit.
You can't do:

void *foo;
// ...
foo += 2.

src/blockdata.c

@@ -193,7 +193,7 @@ void *blockdata_retrieve(struct blockdata *block, size_t len, void *data)
 {
   size_t blen;
   struct  blockdata *b;
-  void *new, *d;
+  uint8_t *new, *d;
   
   static unsigned int buff_len = 0;
   static unsigned char *buff = NULL;