Do not use bzero() for the O_ICMP6TYPE opcode.

The buffer is already zeroed in compile_rule() function, and also it
may contain configured F_NOT flag in o.len field. This fixes the
filling for "not icmp6types" opcode.

Do not print "ip6" keyword in print_icmp6types() for O_ICMP6TYPE opcode.

It produces incompatibility when rules listing is used again to
restore saved ruleset, because "ip6" keyword produces separate opcode.
The kernel already has the check and only IPv6 packets will be checked
for matching.