Core client leaks thread handles on Windows

[Nicolas]

This BOINC client started 47 hours ago (on a reboot). Process Explorer shows boinc.exe has almost 270 thread handles open. Of those, 5 are handles for threads of the client itself. All the rest are invalid handles. Procexp shows lines like "<Non-existent process>(960): 3432", where 960 is the PID (not matching any current process) and 3432 is the thread ID in that process.

This seems to be a handle leak when BOINC starts science apps.

[Nicolas]

Windows API documentation for CreateProcess and CreateProcessAsUser say: "Handles in PROCESS_INFORMATION must be closed with CloseHandle when they are no longer needed."

The PROCESS_INFORMATION structure contains both thread and process handles.

ACTIVE_TASK::cleanup_task in app.cpp is closing the process handle but not the thread handle. In fact, the thread handle is not stored by ACTIVE_TASK::start anywhere, so there is no way cleanup_task could close it.

[Nicolas]

I have only tested 6.2.19, but the relevant code in trunk doesn't seem to have changed, so I assume it happens on all versions.

[davea]

(In [16743]) - client, Win: don't leak thread handlers

fixes #808