Split off init_ifaces from update_iface. init_ifaces discovers the

Split off init_ifaces from update_iface. init_ifaces discovers the
state of the machine on startup using ioctl(2) and getifaddrs(3).
We can then update this state with information provided by route
messages. We still need getifaddrs(3) to check if the layer 2 address
has changed.

This simplifies error handling (what should we do if ioctl(2) fails?),
reduces kernel round trips (no need to ask the kernel again for
information RTM_IFINFO provided already) and prevents a theoretical
race between RTM_IFINFO and getaddrinfo(3).

In a fast link state UP -> DOWN -> UP transition RTM_IFINFO informs us
that the link went down but we were not using this information but
rather looked at getifaddrs(3) information which might see the link as
already up again. We would then do nothing while we should try to get
a new lease.

By storing all interface information in the frontend process we can
skip imsgs to the engine process if we get an RTM_IFINFO without
relevant changes for us.