lib/gpg: Add _FINGERPRINT_PRIMARY to OstreeGpgVerifyResult

Revert the switch of _FINGERPRINT to giving the primary key ID
rather than the signing key ID, and instead add the primary
key ID as a new attribute which is available if the key is not
missing.

Closes: https://github.com/ostreedev/ostree/issues/608

Closes: #1092
Approved by: cgwalters

tests/test-gpg-verify-result.c

@@ -173,7 +173,7 @@ test_attribute_basics (TestFixture *fixture,
       tuple = ostree_gpg_verify_result_get_all (fixture->result, ii);
 
       type_string = g_variant_get_type_string (tuple);
-      g_assert_cmpstr (type_string, ==, "(bbbbbsxxssss)");
+      g_assert_cmpstr (type_string, ==, "(bbbbbsxxsssss)");
 
       /* Check attributes which should be common to all signatures. */