repo: Change GPG verification policy

The global keyring directory (trusted.gpg.d) is deprecated.  Only use it
when a specified remote does NOT have its own keyring, or when verifying
local repository objects.

Note, because mixing in the global keyring directory is now an explicit
choice, OstreeGpgVerifier no longer needs to implement GInitableIface.

src/libostree/ostree-gpg-verifier.h

@@ -37,8 +37,7 @@ typedef struct OstreeGpgVerifier OstreeGpgVerifier;
 
 GType _ostree_gpg_verifier_get_type (void);
 
-OstreeGpgVerifier *_ostree_gpg_verifier_new (GCancellable   *cancellable,
-                                             GError        **error);
+OstreeGpgVerifier *_ostree_gpg_verifier_new (void);
 
 OstreeGpgVerifyResult *_ostree_gpg_verifier_check_signature (OstreeGpgVerifier *self,
                                                              GBytes            *signed_data,
@@ -51,6 +50,10 @@ gboolean      _ostree_gpg_verifier_add_keyring_dir (OstreeGpgVerifier   *self,
                                                     GCancellable        *cancellable,
                                                     GError             **error);
 
+gboolean      _ostree_gpg_verifier_add_global_keyring_dir (OstreeGpgVerifier  *self,
+                                                           GCancellable       *cancellable,
+                                                           GError            **error);
+
 void _ostree_gpg_verifier_add_keyring (OstreeGpgVerifier *self,
                                        GFile             *path);