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
Both methods are now available for Ipv4_address as well as for
Ipv4_address_prefix. An IPv4 address is invalid if it contains zeros only.
An IPv4 address prefix is invalid if its address is invalid and its
prefix is 32.
Ref #2139
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
