libewf.so.2 to libewf.so.3

[Hilko Bengen <bengen@debian.org>]

Hello,

Aleksey Kravchenko and I have been working on updating libewf to from
20140608 to 20171104 to, among other things, gain Python3 bindings.
Unfortunately, there are a few incompatible API/ABI changes which are
correctly marked by a SONAME bump from 2 to 3. This in turn would
normally require a transition because some reverse dependencies (xmount,
guymager, sleuthkit) would need to be rebuilt.

But we are past the transition freeze date for buster. Also, even though
the reverse dependencies are also maintained within this team, at least
sleuthkit would need to be patched and guymager would need to be built
without libewf support.

To avoid changes to reverse dependencies, I'd like to keep a package for
the old version of the shared lib and the headers around. It would look
like this:

src:libewf/20140606-7, stripped-down:
- libewf-dev
- libewf2

src:libewf3/20171104-2, new package for unstable:
- libewf3-dev (Conflicts/Replaces: libewf-dev)
- libewf3
- ewf-tools
- python-libewf
- python3-libewf

Any comments?

Cheers,
-Hilko