Merge branch 'catalog_conf_groups' into 'master'

doc/configuration.rst

doc/man/knot.conf.5in

doc/reference.rst

src/contrib/string.c

src/contrib/string.h

src/knot/catalog/catalog_db.c

src/knot/catalog/catalog_db.h

src/knot/catalog/catalog_update.c

src/knot/catalog/catalog_update.h

src/knot/catalog/generate.c

src/knot/catalog/interpret.c

src/knot/catalog/interpret.h

src/knot/conf/conf.c

src/knot/conf/schema.c

src/knot/updates/zone-update.c

src/knot/zone/zonedb-load.c

src/utils/kcatalogprint/main.c

tests-extra/tests/catalog/groups/data/catalog2.zone

tests-extra/tests/catalog/groups/data/cataloged1.zone

tests-extra/tests/catalog/groups/data/cataloged2.zone