netlink: Improve support for nested Netlink attributes.

netlink: Improve support for nested Netlink attributes.

Fairly often it happens that nested Netlink attributes must themselves
contain Netlink attributes.  In such a case, nlmsg_put_nested() is not so
convenient, because it requires the contents to be pre-assembled and then
copied into place.  This commit introduces a new interface that instead
allows the nested attributes to be assembled in-place.  As a demonstration,
it updates nl_msg_put_nested() to use this new interface.