Add the Mkfifoat function on Linux, akin to the existing Mkfifo function
but taking an additional dirfd parameter. See
http://man7.org/linux/man-pages/man3/mkfifo.3.html

Originally requested in golang/go#14216

Also remove the named result parameter for Mkfifo, as they add no
additional use (suggested by Brad Fitzpatrick).

Change-Id: I7b759752abf3e1469d7f883a7f13313e3062aeb4
Reviewed-on: https://go-review.googlesource.com/48530Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>