Revised MLD-related definitions

• Used mld_xxx and MLD_xxx instead of mld6_xxx and MLD6_xxx according to the official defintions in rfc2292bis (macro definitions for backward compatibility were provided)
• Changed the first member of mld_hdr{} from mld_hdr to mld_icmp6_hdr to avoid name space conflict in C++

This change makes ports/net/pchar compilable again under -CURRENT.

Obtained from: KAME