netdev: Change netdev_get_in4() to return an error code.

netdev: Change netdev_get_in4() to return an error code.

Until now, netdev_get_in4() and netdev_nodev_get_in4() have returned a
bool that represents success or failure.  This commit changes the return
value to an int that can indicate what kind of error occurred, which is
both more consistent with the rest of the netdev interfaces and more
meaningful, and updates all callers to the new interface.

(Currently netdev_get_in4() won't ever return an error, but other future
implementations might.)