fix loading shared library of ICMPv6 match.

fix loading shared library of ICMPv6 match.

The current ip6tables tries to load libip6t_icmp6.so when user types
'ip6tables -p icmpv6 ...' or 'ip6tables ... -m icmpv6' ...', and it fails.

This patch renames libip6t_icmpv6.c to libip6t_icmp6.c so that ip6tables
can load it. Now kernel module and user library has same name 'icmp6'.
It can reduce confusion about name mismatch. That's why I renamed it
instead of reverting change in find_match() which brought this bug.

This patch keeps compatibiity and we can use '-p icmpv6', '-p ipv6-icmpv6',
'-m icmpv6', '-m ipv6-icmpv6', and '-m icmp6', as ever.