src: Revert --literal, add -S/--service

src: Revert --literal, add -S/--service

This is a partial revert of b0f6a45b25dd1 ("src: add --literal option")
which was added during the development cycle before 0.9.1 is released.

After looking at patch: https://patchwork.ozlabs.org/patch/969864/ that
allows to print priority, uid, gid and protocols as numerics, I decided
to revisit this to provide individual options to turn on literal
printing.

What I'm proposing is to provide a good default for everyone, and
provide options to turn on literal/numeric printing.

This patch adds nft_ctx_output_{set,get}_flags() and define two flags to
enable reverse DNS lookups and to print ports as service names.

This patch introduces -S/--services, to print service names as per
/etc/services.

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