nft: Fix command name in ip6tables error message

Upon errors, ip6tables-nft would prefix its error messages with
'iptables:' instead of 'ip6tables:'. Turns out the command name was
hard-coded, use 'progname' variable instead.
While being at it, merge the two mostly identical fprintf() calls into
one.

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