Add env-var support, used by some DBD backends

Some DBD drivers make use of env-vars to control features that are not
adjustable through DSN values. This change supports such configuration.
Add IPv6 columns to track future VPN IPv6 client assignment

This column is currently unused as OpenVPN does not expose client-issued
v6 details in the env-vars available to scripts. When this issue is
fixed (or with a patched OpenVPN version) this project will be updated
to populate this column.

By adding the column now, it avoids the need for a schema update to
support this new feature.