Fix issue #982: Improve detection of Canon lenses sharing the same IDs. Also fixes issues #926, #927, and #938. Thanks goes to Alexander Steffen for the patch.

git-svn-id: svn://dev.exiv2.org/svn@3340 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3
Fix issue #970: Dead code in olympusmn.cpp.

git-svn-id: svn://dev.exiv2.org/svn@3341 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3
I forgot to add error checking in r3341.

git-svn-id: svn://dev.exiv2.org/svn@3342 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3