conf: extend config access flags

src/knot/conf/base.c

src/knot/conf/base.h

src/knot/conf/confdb.c

src/knot/conf/confdb.h

src/knot/conf/confio.c

src/knot/main.c

src/knot/server/server.c

src/libknot/errcode.h

src/libknot/error.c

tests/acl.c

tests/conf.c

tests/confdb.c

tests/confio.c

tests/process_answer.c

tests/process_query.c

tests/requestor.c

tests/test_conf.h