(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.
(maint) move the file mode tests to the files spec module

It is helpful to have a simple helper method to match desired file modes
from spec tests.
(PUP-1177) make sure ssh_known_hosts is not created with mode 0600

The default mode for files that a ParsedFile provider newly creates
is 0600. This makes no sense for the ssh_known_hosts file, because it
contains public information that adds to security for all users.
(PUP-1177) update ParsedFile provider unit tests

The constructor for the :flat filetype accepts a second argument now.
This second argument must be taken into account when stubbing specific
invocations.
(PUP-1177) restore :ram FileType compatibility

The :ram filetype, as an alternative to :flat, is often used for spectests
of ParsedFile providers. The providers normally use the :flat type, but through
mocking, are made to use :ram in test context. Since the providers often pass
both supported arguments to the :flat constructor, it must be accepted by the
constructor of the :ram type as well.