interfaces: switch to Netlink to gather interface-related information

interfaces: switch to Netlink to gather interface-related information

Instead of using getifaddrs() which gives incomplete information, we
use Netlink which is supported since a long time. Many information are
supported by Netlink: bonding, bridging, VLAN... Unfortunately, the
degree of support depends on the version of the kernel:

 - IFLA_VLAN_* stuff is available since 2.6.23 only.
 - IFLA_MASTER seems to be supported since ages (at least
   2.4.32). However, it does not allow to differentiate a bridge and a
   bonding. It does not apply to a VLAN.
 - IFLA_LINK is not sent for a VLAN before 2.6.23.
 - IFLA_BRPORT_* has just landed in Linux.
 - IFLA_LINKINFO and IFLA_INFO_KIND which could help detect the kind
   of device are only available since 2.6.23.

So, in summary, we can't reliably use Netlink to gather VLAN, bridging
and bonding information if we want to support 2.6.18 kernels. When
2.6.23 will be the minimal version, we could do some interesting
things. Also, it seems that ethtool stuff is not available through
Netlink.

This commit also happens to prepare the support of additional OS. Only
interfaces-linux.c (and netlink.c) are now Linux specific. Adding
support for KFreeBSD should be easy enough. More code factorisation
will happen then.