Now that CloudABI's sockets API has been changed to be addressless and
only connected socket instances are used (e.g., socket pairs), they have
become fairly similar to pipes. The only differences on CloudABI is that
socket pairs additionally support shutdown(), send() and recv().
To simplify the ABI, we've therefore decided to remove pipes as a
separate file descriptor type and just let pipe() return a socket pair
of type SOCK_STREAM. S_ISFIFO() and S_ISSOCK() are now defined
identically.
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