Misc. job containers cleanup
Misc. job containers cleanup

This change does two things:

1. Checks if the stdio pipes are nil before closing them via the CloseStdout,
CloseStderr, CloseStdin methods. This just brings it inline with the other
`cow.Process` implementations that check for nilness.
2. Fix an oversight where windows.Close was being used instead of windows.FreeLibrary
after loading kernel32.dll

Signed-off-by: Daniel Canter <8d0fd95ae71f7e4870b010b10f83a6201b034a65@microsoft.com>

Microsoft/hcsshim