Combine redir and masq statements into nat

Combine redir and masq statements into nat

All these statements are very similar, handling them with the same code
is obvious. The only thing required here is a custom extension of enum
nft_nat_types which is used in nat_stmt to distinguish between snat and
dnat already. Though since enum nft_nat_types is part of kernel uAPI,
create a local extended version containing the additional fields.

Note that nat statement printing got a bit more complicated to get the
number of spaces right for every possible combination of attributes.

Note also that there wasn't a case for STMT_MASQ in
rule_parse_postprocess(), which seems like a bug. Since STMT_MASQ became
just a variant of STMT_NAT, postprocessing will take place for it now
anyway.

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