When other end of the pipe closed during the write, but some bytes
were written, return short write instead of EPIPE.

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Discussed with:	bde (long time ago)
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274023 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f