bsd: separate network unrelated code from net_init()

It's strange for net_init() to initialize things like physmem and ZFS.
Create a function bsd_init() for initializing network unrelated components.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Zifei Tong <zifeitong@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>