Apply the style rule that an accessor is named similar to the the underlying
value. Provide read and write accessors for each mandatory header attribute.
Fix some incorrect structure in the headers like with the flags field
in Ipv4_packet.
Ref #2490
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