xtables-translate: Leverage stored protocol names

xtables-translate: Leverage stored protocol names

Align output of ip(6)tables-translate for --protocol arguments with that
of ip(6)tables -L/-S by calling proto_to_name() from xshared.c. The
latter will consult xtables_chain_protos list first to make sure (the
right) names are used for "common" protocol values and otherwise falls
back to getprotobynumber() which it replaces here.

Link: https://bugzilla.netfilter.org/show_bug.cgi?id=1738
Signed-off-by: Phil Sutter <phil@nwl.cc>