examples: fix some compiler warnings

docs/examples/anyauthput.c

@@ -100,7 +100,7 @@ static curlioerr my_ioctl(CURL *handle, curliocmd cmd, void *userp)
 /* read callback function, fread() look alike */
 static size_t read_callback(void *ptr, size_t size, size_t nmemb, void *stream)
 {
-  size_t retcode;
+  ssize_t retcode;
   curl_off_t nread;
 
   intptr_t fd = (intptr_t)stream;

docs/examples/sendrecv.c

@@ -98,7 +98,7 @@ int main(void)
       return 1;
     }
 
-    sockfd = sockextr;
+    sockfd = (curl_socket_t)sockextr;
 
     /* wait for the socket to become ready for sending */
     if(!wait_on_socket(sockfd, 0, 60000L)) {

docs/examples/sepheaders.c

@@ -31,7 +31,7 @@
 
 static size_t write_data(void *ptr, size_t size, size_t nmemb, void *stream)
 {
-  int written = fwrite(ptr, size, nmemb, (FILE *)stream);
+  size_t written = fwrite(ptr, size, nmemb, (FILE *)stream);
   return written;
 }