(maint) Reduce amount of stubbing in init_spec

It seems to be easier to work with real objects.
(maint) Remove redundant code in init provider

The test `!File.directory?` is redundant because if the item actually is
a directory, we cannot reach that statement.
(maint) Be more explicit in init_spec expectations

Instead of stubbing every call to File.directory? that may have side
effects in other areas of puppet be more explicit in what we actually
expect.

This may also prevent tests to pass incidentally.
(maint) Simplify init provider instances tests

Let the instances method create real provider instances and just check
the returnvalue
(#14615) Move exclude list into the init provider

Some initscripts are not save to execute on different systems. Example:
If you run `/etc/init.d/reboot.sh status` on a gentoo system you will
immediatly reboot the system.

This can be a problem if you run `puppet resource service` as puppet
will try to get a list of services and then runs the status command on
all of them.

The issue was first addressed in redmine ticket #14615 and #14761 and
introduced different exclude lists for the gentoo provider and the
redhat provider. Unfortunately this did not resolve the problem because
when running `puppet resource service` puppet will not just query the
most suitable provider for a list of services but all suitable
providers. So if the gentoo provider now hides the `reboot.sh` script,
but the init provider (which is also suitable) does not, puppet will
ask the init provider to get the status of the service. As a result
the init provider will now happily reboot the system.

Move the exclude lists into the init provider so the init provider does
the filtering. Each provider that uses the `init` provider as its parent
does not have to do any filtering anymore.

This fix assumes that an initscript that has to be excluded on one
system is never a valid initscript on another system. Given the recent
list of excludes (reboot.sh, shutdown.sh, etc) this seems valid
Merge pull request #1473 from stschulte/ticket/master/initprovider_shutdowns_system

Do not reboot system when runnnig puppet resource service