(#7244) Consolidate autosign setting

Previous implementation split out a new autosign_command setting for the
custom policy executable, which required a confusing autosign=false
setting.

This commit removes the autosign_command setting and instead uses the
autosign setting for all modes (true/false, autosign.conf or custom
policy executable).  Internally we refactor to distinguish between
autosign.conf or an executable by the executable bit.

We are removing the mode of the autosign setting, because it was
stripping all executables of their executable bit.  This only removes
the guarantee that group and other do not have write permissions.
However, these are the default permissions for a regular file on most
systems, so nothing should be lost.
(Maint) Introduce partial implementation of an In-Memory file

Quite often in tests we want to work with a "file", but not actually
deal with the file system. We usually do that by mocking out all sorts
of things on File and FileUtils and Dir and various other places. Now
that we have Puppet::FileSystem::File (and maybe at some point in the
future we'll have Puppet::FileSystem::Store to represent the actual
storage system, what we normally call the filesystem itself) we can
create different kinds of files.

This adds Puppet::FileSystem::MemoryFile, which, so far, only partially
implements the methods on Puppet::FileSystem::File and addes 3 methods
to Puppet::FileSystem::File to allow us to easily create a view onto the
filesystem: overlay, remember, forget.
(#7244) Fix up tests after refactors of the Autosign system

The refactoring of the Autosign system changed its interaction with the
filesyste. This fixes up the tests to work with the new interactions and
hopefully be more robust to future changes.
Merge pull request #2083 from zaphod42/jpartlow-feature/7244/refactor-autosign-setting

(#7244) Consolidate autosign setting