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.

examples/client.c

@@ -194,7 +194,7 @@ coap_new_request(coap_context_t *ctx,
       unsigned char buf[4];
       coap_add_option(pdu,
                       COAP_OPTION_SIZE1,
-                      coap_encode_var_safe(buf, sizeof(buf), length),
+                      coap_encode_var_safe8(buf, sizeof(buf), length),
                       buf);
 
       coap_add_block(pdu, length, data, block.num, block.szx);
@@ -314,7 +314,7 @@ resolve_address(const coap_str_const_t *server, struct sockaddr *dst) {
     switch (ainfo->ai_family) {
     case AF_INET6:
     case AF_INET:
-      len = ainfo->ai_addrlen;
+      len = (int)ainfo->ai_addrlen;
       memcpy(dst, ainfo->ai_addr, len);
       goto finish;
     default:
@@ -572,7 +572,7 @@ message_handler(struct coap_context_t *ctx,
 
           coap_add_option(pdu,
                           COAP_OPTION_SIZE1,
-                          coap_encode_var_safe(buf, sizeof(buf), payload.length),
+                          coap_encode_var_safe8(buf, sizeof(buf), payload.length),
                           buf);
 
           coap_add_block(pdu,
@@ -906,7 +906,7 @@ set_blocksize(void) {
     opt = method == COAP_REQUEST_GET ? COAP_OPTION_BLOCK2 : COAP_OPTION_BLOCK1;
 
     block.m = (opt == COAP_OPTION_BLOCK1) &&
-      ((1u << (block.szx + 4)) < payload.length);
+      ((1ull << (block.szx + 4)) < payload.length);
 
     opt_length = coap_encode_var_safe(buf, sizeof(buf),
           (block.num << 4 | block.m << 3 | block.szx));
@@ -1078,7 +1078,7 @@ decode_segment(const uint8_t *seg, size_t length, unsigned char *buf) {
 static int
 check_segment(const uint8_t *s, size_t length) {
 
-  size_t n = 0;
+  int n = 0;
 
   while (length) {
     if (*s == '%') {
@@ -1458,7 +1458,7 @@ get_session(
       coap_address_t bind_addr;
       if ( rp->ai_addrlen <= sizeof( bind_addr.addr ) ) {
         coap_address_init( &bind_addr );
-        bind_addr.size = rp->ai_addrlen;
+        bind_addr.size = (socklen_t)rp->ai_addrlen;
         memcpy( &bind_addr.addr, rp->ai_addr, rp->ai_addrlen );
         session = open_session(ctx, proto, &bind_addr, dst,
                                identity, identity_len, key, key_len);

examples/coap-rd.c

@@ -723,7 +723,7 @@ get_context(const char *node, const char *port) {
 
     if (rp->ai_addrlen <= sizeof(addr.addr)) {
       coap_address_init(&addr);
-      addr.size = rp->ai_addrlen;
+      addr.size = (socklen_t)rp->ai_addrlen;
       memcpy(&addr.addr, rp->ai_addr, rp->ai_addrlen);
       addrs = addr;
       if (addr.addr.sa.sa_family == AF_INET) {

examples/coap-server.c

@@ -35,6 +35,8 @@ static char* strndup(const char* s1, size_t n)
   }
   return copy;
 };
+#include <io.h>
+#define access _access
 #else
 #include <unistd.h>
 #include <sys/select.h>
@@ -320,10 +322,11 @@ check_async(coap_context_t *ctx,
             coap_tick_t now) {
   coap_pdu_t *response;
   coap_async_state_t *tmp;
+  async_data_t data = { .ptr = async ? async->appdata : NULL };
 
   size_t size = 13;
 
-  if (!async || now < async->created + (unsigned long)async->appdata)
+  if (!async || now < async->created + data.val)
     return;
 
   response = coap_pdu_init(async->flags & COAP_ASYNC_CONFIRM
@@ -332,7 +335,6 @@ check_async(coap_context_t *ctx,
              COAP_RESPONSE_CODE(205), 0, size);
   if (!response) {
     coap_log(LOG_DEBUG, "check_async: insufficient memory, we'll try later\n");
-    async_data_t data = { .ptr = async->appdata };
     data.val = data.val + 15 * COAP_TICKS_PER_SECOND;
     async->appdata = data.ptr;
     return;
@@ -1075,7 +1077,7 @@ get_context(const char *node, const char *port) {
 
     if (rp->ai_addrlen <= sizeof(addr.addr)) {
       coap_address_init(&addr);
-      addr.size = rp->ai_addrlen;
+      addr.size = (socklen_t)rp->ai_addrlen;
       memcpy(&addr.addr, rp->ai_addr, rp->ai_addrlen);
       addrs = addr;
       if (addr.addr.sa.sa_family == AF_INET) {