We have a cached nm_device_get_ip_iface() property. However, the interface
name is not an identifier for a link because it can change at any time.

Also, we already have the (ip) ifindex as proper identifier for the
platform link. We shouldn't use two redundant identifiers to refer to
a link.

Clearly, sometimes we need an ifname. For example for ethtool ioctl or
sysctl path names. For ethtool API, we resolve the actual name as late
as possible, and for sysctl API we prefer NMP_SYSCTL_PATHID_NETDIR*().
However, that is not always possible, for example for /proc/sys/net/ipv6/conf/
sysctls.

Add a function that resolves the ifname by looking into the cache. This
of course is still racy, but it minimizes the time.

Also, we should less and less rely on the ifname, and resolve it as late
as possible. This patch adds a small wrapper going into that direction.