DHCPv6: axe a couple invariant expressions

The "type > 65535" condition in dhcp6opt_name() was always false because
the function is given a 16-bit argument.

The "dh6->dh6_msgtype" condition in dhcp6_print() was always true
because name == NULL only when none of the preceding switch block cases
matched.
DHCPv6: make use of tok2str()