src: Add support for NFTNL_SET_DESC_CONCAT

src: Add support for NFTNL_SET_DESC_CONCAT

To support arbitrary range concatenations, the kernel needs to know
how long each field in the concatenation is. The new libnftnl
NFTNL_SET_DESC_CONCAT set attribute describes this as an array of
lengths, in bytes, of concatenated fields.

While evaluating concatenated expressions, export the datatype size
into the new field_len array, and hand the data over via libnftnl.

Similarly, when data is passed back from libnftnl, parse it into
the set description.

When set data is cloned, we now need to copy the additional fields
in set_clone(), too.

This change depends on the libnftnl patch with title:
  set: Add support for NFTA_SET_DESC_CONCAT attributes

v4: No changes
v3: Rework to use set description data instead of a stand-alone
    attribute
v2: No changes

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>