Expose the existing experimental Packet Burst code through a CMake option.
Generate ENABLE_BURSTMODE in config.h from -DENABLE_BURSTMODE instead of
keeping it hard-coded to 0, and print the selected state during configuration.
Also fix the existing nwconn burst dispatch conditional so the source compiles
when ENABLE_BURSTMODE is enabled.
This only makes the existing Packet Burst code build-selectable; it does not
change the default behavior, which remains disabled.
