asyn: use 'struct thread_data *' instead of 'void *'

To reduce use of types that can't be checked at compile time. Also
removes several typecasts.

... and rename the struct field from 'os_specific' to 'tdata'.

Closes #6239
Reviewed-by: Jay Satiro

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/urldata.h b/lib/urldata.h
index 805957153..f085c093c 100644
--- a/lib/urldata.h
+++ b/lib/urldata.h
@@ -519,7 +519,7 @@ struct Curl_async {
   int port;
   struct Curl_dns_entry *dns;
   int status; /* if done is TRUE, this is the status from the callback */
-  void *os_specific;  /* 'struct thread_data' for Windows */
+  struct thread_data *tdata;
   BIT(done);  /* set TRUE when the lookup is complete */
 };