fstab-generator: Apply _netdev option also to device units

In case the device field of fstab record is an actual device (not an address)
apply same dependencies to the device unit as to the mount unit, i.e.

> After=network-online.target network.target
> Wants=network-online.targe

It makes sense to start the device expecting job only when network is actually
ready (consider e.g. iSCSI devices) since it is device's implicit dependency.

The eventual implementation should better obtain network flag from udev
database and would also take into account device hierarchy (see [1]).
This patch approximates that by taking the `_netdev` option as a hint from the
user both about the filesystem and underlying device. (For local devices with
network filesystems (e.g. ocfs2), this hint leads to unused dependencies.)

[1] https://lists.freedesktop.org/archives/systemd-devel/2014-October/024718.html