Adds sys/class/net devices to linsysfs.

Only two interfaces are created eth0 and lo and they expose
the following properties:
address, addr_len, flags, ifindex, mty, tx_queue_len and type.

Initial patch developed by Carlos Neira in 2017 and finished by me.

PR: 223722
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D13656