Added handle_t type to server interface so that we can make handles
pointers later on.
Always use 0 to signal invalid handle in server requests.

diff --git a/dlls/kernel/toolhelp.c b/dlls/kernel/toolhelp.c
index fccb241..c2486dc 100644
--- a/dlls/kernel/toolhelp.c
+++ b/dlls/kernel/toolhelp.c
@@ -212,6 +212,7 @@
         ret = req->handle;
     }
     SERVER_END_REQ;
+    if (!ret) ret = INVALID_HANDLE_VALUE;
     return ret;
 }