NetBSD /dev/fuse detection

NetBSD's FUSE being pure userland implementation, there is no /dev/fuse
to open. Test /dev/puffs (kernel fs-in-userland subsystem supporting FUSE)
insead.

BUG: 764655
Change-Id: Ia65e95c246dc31ea2839cf64d7c851430828542e
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8478
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>