Clean gdaldrivermanager.cpp.

- Add cpl_port include
- Add a C++11 mutex lock_guard to GDALDestroyDriverManager when using C++11
    Experiment with C++11 mutexes
- Initialize vars
- Convert commented out CPLDebug into a DEBUG_VERBOSE conditional
- C casts -> C++ casts
- Remove extra parens
- Add const to local vars
- iFoo++ -> ++iFoo; iFoo-- -> --iFoo
- /* */ -> for non-block comments
- Formatting
- Drop cast to void * types


git-svn-id: https://svn.osgeo.org/gdal/trunk@34185 f0d54148-0727-0410-94bb-9a71ac55c965