netfilter support in libnl

[Philip Craig <philipc@snapgear.com>]

There have been comments in the past on this list about using libnl
for the netfilter netlink support, and since I would like to use a
common library api for all the netlink interfaces, I decided to give
it a go.  These patches are the result of that.

Any comments welcome, particularly about whether this is still
something that is desired.

Some notes about the patches:
- only tested on an ARM platform
- only conntrack and log implemented (not queue)
- only dumping and monitoring implemented (no modifications)
- monitoring ct/log events requires copying all the netlink message
data into another object, which seems a bit inefficient
- I defined a cache to monitor log messages, so that it can find the
parser automatically, but not sure that caching log events makes sense
- I've added 'test' operations for some ct/log fields.  None of the
other objects in libnl have these, but I think they are needed.