Reuse sock.c logic in mbedtls integration

src/sock.c

@@ -75,7 +75,8 @@ static void tomgaddr(union usa *usa, struct mg_addr *a, bool is_ip6) {
 #endif
 }
 
-static bool mg_sock_would_block(void) {
+bool mg_sock_would_block(void);
+bool mg_sock_would_block(void) {
   int err = MG_SOCK_ERRNO;
   return err == EINPROGRESS || err == EWOULDBLOCK
 #ifndef WINCE