idmap2: fix cpplint warnings

Fix the following to make cpplint happy again:

  - Use C++ style casts
  - Rename a few header guards
  - Add missing includes of standard C++ headers
  - Add comments to the end of namespaces

Also, for consistency, always use "..." instead of <...> for local
include files, i.e.

  #include "idmap2/.*"

Test: cmds/idmap2/static-checks.sh
Change-Id: Ie735a36d562d1fc51b61f1f0f45aec0d160f602d
7 files changed