So although the poll() implemenation is still not yet usable, the commit is useful because it (1) does not harm, and (2) incidentally fixes a few other issues in the NETLONK response queuing that I noted in the process.
Squashed commit of the following:
net/netlink: Mark netlink support as EXPERIMENTAL.
net/netlink/netlink_sockif.c: Add netlink_getpeername to the socket interface.
net: Add getpeeername() support for netlink sockets.
include/netpacket/netlink.h: Add a few more definitions and structures used at the NetLink interface. Still missing many.
net/netlink: Add basic framework for Netlink socket support.
include/: Add basic Netlink definitions.
