src: backport ignore ENOTCONN on shutdown race

This is a backport of ea37ac04



Original commit message:

  On AIX, OS X and the BSDs, calling shutdown() on one end of a pipe
  when the other end has closed the connection fails with ENOTCONN.

  The sequential/test-child-process-execsync test failed sporadically
  because of a race between the parent and the child where one closed
  its end of the pipe before the other got around to calling shutdown()
  on its end of the pipe.

  Libuv is not the right place to handle that because it can't tell if
  the ENOTCONN error is genuine but io.js can.

  Refs: libuv/libuv#268
  PR-URL: iojs#1214
Reviewed-By: Bert Belder <bertbelder@gmail.com>

Fixes: https://github.com/joyent/node/issues/9444

.

Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
PR-URL: https://github.com/joyent/node/pull/14480