(PUP-1177) allow parsedfile providers to choose a default mode

The ParsedFile provider baseclass and its companion Util::FileType module
employ Tempfile for pre-buffering generated files. If there is no actual
file at the final desination yet, copying the tempfile will preserve its
permissions, which implies a mode of 0600.

This is not desirable for all files managed by ParsedFile providers. Therefor,
the ParsedFile provider baseclass now provides an interface to change this
default mode by overriding a method.