src: ignore ENOTCONN on shutdown race with child

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: https://github.com/libuv/libuv/pull/268
PR-URL: https://github.com/iojs/io.js/pull/1214


Reviewed-By: Bert Belder <bertbelder@gmail.com>