MFC r326437:

Correct history for Unix 2nd Edition through 6th Edition for the
system calls. Man pages are missing for v2 and v5, so any entries for
those versions were inferred by new implementations of these functions
in libc.

MFC r303104, r303106:

Update to reflect the fact that pipe() is a wrapper around the pipe2()
system call.

Reviewed by: jhb, wblock
Sponsored by: DAPRA, AFRL

Change wording to use function rather than system call in the
description
as well.

Authored by: zeising
Reviewed by: brooks

Approved by: re (gjb)
Sponsored by: DAPRA, AFRL