Fix the implementation of closewrite(::AbstractPipe)

[JamesWrigley]

Previously this would just call closewrite(p.in), which would throw an exception since closewrite isn't supported for the libuv stream underlying PipeEndpoint.

This fixes the example in the Pipe docstring: https://docs.julialang.org/en/v1/base/io-network/#Base.Pipe

[JamesWrigley]

AFAICT the test and build failures are unrelated.

[vtjnash]

The function close is not generally equivalent to closewrite, so it cannot just replace one with the other. We can potentially make closewrite more robust however

[JamesWrigley]

I'm not sure I follow, isn't closing the write end of a Pipe exactly what closewrite should do? What else would need to be done?

[vtjnash]

The shutdown syscall has expected side-effects whenever the user has a socket there (sends an EOF explicitly, even if other kernel handles exist) and doesn't destroy the read side of the handle if that happens to also be present