Change naming convention for C++ interfaces

Change naming convention for C++ interfaces

- Describe the new convention in naming.rst, together with some
  rationale.  The new I prefix is a very common convention, and makes
  the code generally more readable than a long Interface suffix that
  does not add much information.
- Do a bulk rename from \w*Interface to I\1, skipping
  ::testing::WithParamInterface.
- Rename helptopicinterface.h to ihelptopic.h to keep the naming
  consistent.
- Run uncrustify on the result.

Change-Id: I4b27ae0164ad89e45693d35055730caab0832bed