BSD: Fix UP/DOWN for interfaces which dont' report media changes

BSD: Fix UP/DOWN for interfaces which dont' report media changes

Carrier UP/DOWN state is handled by media change events on BSD.
When the carrier state changes, it's always to LINK_STATE_DOWN or
LINK_STATE_UP.
If we receive LINK_STATE_UNKNOWN from RTM_IFINFO it means the
interface doesn't change media change events and as such won't
report carrier state changes. In this instance, we can only rely
on IFF_UP being set, which is the same condition dhcpcd needs
for LINK_STATE_UP.