Implement persist mode for netlink

There is a "-p" mode for ioctl connections, which attempts to reconnect
to the server if the connection dies. It doesn't really work anymore
though.

The netlink interface has a "monitor" message that can get sent to a
multicast group, and a "reconfigure" message that can replace dropped
connections. Additionally, there is a "dead link timeout" option which
can be used to tell the kernel to block I/O to a device for X amount of
time.

The three combined allow for a much more flexible way of persist mode
using netlink.

Implement that, and a test for it.

TODO: test. It compiles, and seems to behave correctly against our
mocked libnl3, that's all I can say for now, but no guarantees (yet)
that it will actually work correctly against a kernel and/or an
unreliable network.

Signed-Off-By: Wouter Verhelst <w@uter.be>

Author: yoe