xshared: Share print_iface() function

xshared: Share print_iface() function

Merge the three identical copies into one and name it 'save_iface' (as
the printed syntax is for "save"-format). Leave arptables alone for now,
its rather complicated whitespace printing doesn't allow for use of the
shared function. Also keep ebtables' custom implementation, it is used
for the --logical-in/--logical-out long-options, too. Apart from that,
ebtables-nft does not use a mask, at all.

Signed-off-by: Phil Sutter <phil@nwl.cc>