Avoid double appearing of cloned interfaces in the output
from list_net_interfaces() when network_interfaces=auto.

Rationale: Since the auto case is special, the lesser evil
had to be chosen among not adding cloned interfaces to
_tmplist or removing duplicates from _tmplist after adding
cloned interfaces. Since list_net_interfaces() must not use
/usr/bin tools, the former "evil" appeared clearer and much
more efficient. (See the PR audit trail for discussion.)

PR: conf/63700
Reviewed by: brooks
MFC after: 5 days