This moves getethertype.c into libxtables so that both extensions and
xtables-nft-multi may use the implementations therein. New users are
libebt_arp and libebt_vlan which drop their own duplicated
implementations of getethertypebyname() for the shared one.
This change originated from a covscan report of extensions'
implementations not checking fopen() return value which should be
implicitly fixed by this as well.
Signed-off-by: Phil Sutter <email@example.com>
Signed-off-by: Florian Westphal <firstname.lastname@example.org>