Checking sendfile() availability in configure.
Checking sendfile() availability in configure.

Removing SF_NODISKIO flag for FreeBSD sendfile() check because it
is not used yet and to support DragonFlyBSD.

This closes #414 issue on GitHub.

nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit