XDP-TCP: configurable parameters of TCP stack

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/server/server.c

src/knot/server/udp-handler.c

src/knot/server/xdp-handler.c

src/knot/server/xdp-handler.h

tests/knot/test_confio.c