Windows C4267 errors: Fix size_t conversion warnings in windows builds

examples/client.c:
examples/coap-rd.c:
examples/coap-server.c:
include/coap2/block.h:
include/coap2/coap_hashkey.h:
src/block.c:
src/coap_debug.c:
src/coap_hashkey.c:
src/coap_io.c:
src/coap_openssl.c:
src/net.c:

Fix C4267 windows compilation warnings when using Visual Studio where
size_t variable parameters are being converted  in both size and sign.

There still are some C4116 warnings to fix.

include/coap2/block.h

@@ -133,7 +133,7 @@ int coap_write_block_opt(coap_block_t *block,
  * @return          @c 1 on success, @c 0 otherwise.
  */
 int coap_add_block(coap_pdu_t *pdu,
-                   unsigned int len,
+                   size_t len,
                    const uint8_t *data,
                    unsigned int block_num,
                    unsigned char block_szx);