Fix strange names and consistency issues in API names

[sethmlarson]

Things like dn_d_mode are bad API for those that don't know what they're looking for. Should categorize all the instances of this here.

• i_pv6 -> ipv6
• i_pv4 -> ipv4
• ip6 -> ipv6
• port_forward_rules4 -> port_forward_rules_ipv4
• port_forward_rules6 -> port_forward_rules_ipv6
• dn_d_mode -> drag_and_drop_mode

[sethmlarson]

This has been completed in the 2.0.0 branch