knotd,kxdpgun: adapt to new XDP initialization

doc/man/knot.conf.5in

doc/reference.rst

src/knot/conf/base.c

src/knot/conf/base.h

src/knot/conf/schema.c

src/knot/conf/schema.h

src/knot/conf/tools.c

src/knot/server/server.c

src/knot/server/xdp-handler.c

src/utils/kxdpgun/main.c

tests/knot/test_confio.c