Fix cleanup code

[rzhao271]

Fixes #512

[rzhao271]

I have confirmed that I have moved the code to a part of the file that is called later:

• WindowsPtyAgent#ctor calls into PtyStartProcess and PtyConnect
• PtyConnect sets up a process exit listener that calls OnProcessExit
• PtyKill closes the hShell handle, but OnProcessExit is the part of the code that actually reads hShell's exit code, so I'm thinking that OnProcessExit is always run later than PtyKill

I still have to test this PR, though.

[rzhao271]

I have tested the PR
DisconnectNamedPipe causes the crash. I have removed those calls for now and added a comment referencing the issue.

Edit: DisconnectNamedPipe fails with error ERROR_INVALID_HANDLE. I wonder whether that means that after pfnClosePseudoConsole is called, the hIn and hOut handles are somehow invalidated?

Edit: I ran another experiment where I moved DisconnectNamedPipe back to the PtyKill implementation, and I got the error ERROR_ACCESS_DENIED.