Unify usage and API of CppCheck class. Allow only one file checked
at a time, instead of list of files. Clients can then handle file
lists more naturally and as they see fit. Also clients have better
knowledge of how checking status should be handled.
The single-threaded CLI checking was only one using the file list.
Other clients were giving files (to list) one file at a time.
Greg Hewgill
Kimmo Varis
Daniel Marjamäki