Clear userWantsWrite before calling onStdinReady. (#112)

[bturner]

Prior to this change, if wantWrite() was called from another thread while NuProcess was calling onStdinReady(ByteBuffer), that request would be "lost" if onStdinReady returned false. Depending on where input is coming from (for example a Servlet 3.1 ReadListener), that could result in the process getting "stuck" indefinitely.

• Updated BasePosixProcess.writeStdin(int, int) to clear the userWantsWrite flag, if it's set, prior to calling onStdinReady, and then only re-set userWantsWrite if onStdinReady returns true
• Updated WindowsProcess.writeStdin(int) to use the same technique
  • Also fixed an unrelated bug where checkHandleValidity was comparing the provided HANDLE to the invalid handle's pointer, rather than comparing pointers from both
• Added a new unit test that verifies the wantWrite() request isn't "lost" if it happens during an onStdinReady callback
  • This test fails on Linux, macOS and Windows without the changes to writeStdin in BasePosixProcess and WindowsProcess

[bturner]

Thanks for the quick merge and release, @brettwooldridge. I appreciate all your support.

[brettwooldridge]

@bturner No, thank you for the contributions.