libgudev is just a wrapper around libudev. We can
use libudev directly and drop the dependency for
libgudev.

We call nmp_utils_ethtool_get_driver_info() twice when receiving a
netlink message, but we don't need a clone of the string values.
Instead, expose a data structure that should be stack allocated
by the caller.

The ioctl APIs ethtool/mii require an interface ifname. That is inherrently
racy as interfaces can be renamed. This cannot be fixed, we can only
minimize the time between verifying the ifname and calling ioctl.

We already had problems with that when ethtool would access an interface
by name that didn't exists. See commit ab41c13b0611c6cc967b055d328637a143b5c59b .
Checking for an existing interface only helps avoiding races when an interface
gets deleted. It does not help against renaming.

Go one step further, and instead of checking whether such an ifname
exists, try to get the ifname based on the ifindex immediately before
we need it.

This brings an additional overhead for each ethtool access.

- use nm_auto_close cleanup attribute
- optionally, return the found ifname
- don't stat "phy80211". If such an entity can be opened,
  just assume it's a directory.