Add CompatUtility::IsLegacyAttribute().

Refs #5855
DB IDO: Dump application and command vars.

Refs #5855
Livestatus: Add command custom variables.

Refs #5855
StatusDataWriter: Dump command and application custom variables.

Refs #5855
Update documentation with schema extensions.

Refs #5855
Livestatus: Add application variables to status table.

Refs #5855