core: introduce PinnedResource

This introduces PinnedResources as a structure combining pinned
references to a root directory, root image, or root mstack. This is not
only easier to work with, but essential to make certain unpriv things
work, as we need some mechanism to pin resources before we drop into a
userns which might possibly not provide access anymore to those
resources.

Hence this does two things: introduce the new structure, and immediately
hook it up so that we pin things properly before dropping into userns,
and then makes use of this after dropping the right way, and enables
unpriv userns operation.

The concept is generic enough to eventually implement extension images +
mount images with the same structure, but in order to keep the changes
managable this is left for another time.

(This also makes one further clean-up: client-side verity-reuse checks
are moved server side if we are unpriv. Previously we'd do them client
side, but they were doomed to fail because of lack of privs. Hence let's
drop the client side if we are unpriv and purely do them server-side in
that case.)

src/shared/shared-forward.h

@@ -74,6 +74,7 @@ typedef struct LoopDevice LoopDevice;
 typedef struct MachineBindUserContext MachineBindUserContext;
 typedef struct MachineCredentialContext MachineCredentialContext;
 typedef struct MountOptions MountOptions;
+typedef struct MStack MStack;
 typedef struct OpenFile OpenFile;
 typedef struct Pkcs11EncryptedKey Pkcs11EncryptedKey;
 typedef struct Table Table;