subnet prefix ("subnet" parameter) ambiguity

Currently you can create two subnets with the same [begin,end) prefix as soon as the "subnet" parameter is not the same. Some customers already use this trick so create "192.0.2.0/24", "192.0.2.1/24", etc, subnets.

There are three solutions:

• ignore (this is what we did not it is not a real solution!)

• make this illegal (ref #36 (closed) and #37)

• accept the trick

In the last case we should create a ticket to add unit tests (so if a code change makes it no longer to work it will be detected) and document it.

BTW the criterion to choose between to make illegal or to accept is the use of the prefix as an unique key in config backend database or things like the remote-subnet4-del-prefix CB command which makes sense only if a prefix can identify without ambiguity a subnet. So it is a design choice and IMHO it must be explicitly done.