netfilter: nftables: generalize set extension to support for several expressions

This patch replaces NFT_SET_EXPR by NFT_SET_EXT_EXPRESSIONS. This new
extension allows to attach several expressions to one set element (not
only one single expression as NFT_SET_EXPR provides). This patch
prepares for support for several expressions per set element in the
netlink userspace API.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>