chain: Hash chain list by name

Introduce a hash table to speedup nftnl_chain_list_lookup_byname(). In
theory this could replace the linked list completely but has been left
in place so that nftnl_chain_list_add_tail() still does what it's
supposed to and iterators return chains in original order.

Speed was tested using a simple script which creates a dump file
containing a number of custom chains and for each of them two rules in
INPUT chain jumping to it. The following table compares run-time of
iptables-legacy-restore with iptables-nft-restore before and after this
patch:

count      legacy       nft-old        nft-new
----------------------------------------------
10000         26s           38s            31s
50000        137s          339s           149s

So while it is still not as quick, it now scales nicely (at least in
this very primitive test).

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

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