libnftables: Store top_scope in struct nft_ctx

libnftables: Store top_scope in struct nft_ctx

Allow for interactive sessions to make use of defines. Since parser is
initialized for each line, top scope defines didn't persist although
they are actually useful for stuff like:

| # nft -i
| define goodports = { 22, 23, 80, 443 }
| add rule inet t c tcp dport $goodports accept
| add rule inet t c tcp sport $goodports accept

While being at it, introduce scope_alloc() and scope_free().

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>