Regenerate z-files for DragonFly BSD 3.6.

F_DUP_FD_CLOEXEC is now supported, so remove the zero value constant
from types_dragonfly.go so that we use the generated value from the
z-files.

LGTM=mikioh.mikioh
R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/70080047