Rename GPL headers in cpp/h files

include/clinkahead.h

include/linkahead/acm/user.h

include/linkahead/authentication.h

include/linkahead/certificate_provider.h

include/linkahead/configuration.h

include/linkahead/connection.h

include/linkahead/constants.h.in

include/linkahead/data_type.h

include/linkahead/entity.h

include/linkahead/exceptions.h

include/linkahead/file_descriptor.h

...de/linkahead/file_transmission/download_request_handler.h

include/linkahead/file_transmission/file_error.h

include/linkahead/file_transmission/file_reader.h

include/linkahead/file_transmission/file_writer.h

...inkahead/file_transmission/register_file_upload_handler.h

include/linkahead/file_transmission/upload_request_handler.h

include/linkahead/handler_interface.h

include/linkahead/info.h

include/linkahead/log_level.h