In other contexts (IPv4) 'calc_checksum' merely returns a new checksum that is
then installed via 'checksum'. Thus, the UDP 'calc_checksum' that calculates AND
installs a new checksum is misleading.
Ref #114
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082