Port code from the KWSys System_Shell APIs into cmOutputConverter.
Drop it from our copy of KWSys because upstream will drop it too,
and by doing it in this commit 'git blame' may have an easier time
connecting the history of the content.
KWSys upstream added an attribute to export .gitattributes so that
importing snapshots into the sources of other projects would bring along
the attributes. However, we don't want to export them from CMake.
Drop .gitattributes entries not relevant to CMake.
