w32: Cleanup use of pid_t in call-daemon

* agent/call-daemon.c (struct wait_child_thread_parm_s) [W32]: Do not
use HANDLE for pid_t.
(wait_child_thread): Ditto.

Mingw has its own definition of pid_t as does libassuan. We should use
this instead of using HANDLE. Things are a bit complicated in
Windows, because Windows also has a pid_t but that is mostly useless;
in particular because you can't wait on a real pid but need a handle.