Simplify xbps_transaction_prepare()/commit() and related API changes.

- xbps_handle::transd -> new member with transaction dictionary.
- xbps_transaction_prepare: returns an int.
- xbps_transaction_commit: doesn't need any arg now.
- xbps_repository_pool_sync: doesn't need any arg now.
- xbps_pkgdb_update: removed xbps_handle * arg.
- xbps_transaction_missingdeps_get: removed, missing_deps array is in
  xbps_handle::transd("missing_deps") array object.

include/xbps_api_impl.h

@@ -120,8 +120,7 @@ int HIDDEN xbps_remove_obsoletes(const char *,
  * @private
  * From lib/repository_finddeps.c
  */
-int HIDDEN xbps_repository_find_pkg_deps(prop_dictionary_t,
-					 prop_array_t,
+int HIDDEN xbps_repository_find_pkg_deps(struct xbps_handle *,
 					 prop_dictionary_t);
 
 /**
@@ -135,13 +134,13 @@ int HIDDEN xbps_requiredby_pkg_remove(const char *);
  * @private
  * From lib/transaction_sortdeps.c
  */
-int HIDDEN xbps_sort_pkg_deps(void);
+int HIDDEN xbps_transaction_sort_pkg_deps(struct xbps_handle *);
 
 /**
  * @private
  * From lib/transaction_dictionary.c
  */
-prop_dictionary_t HIDDEN xbps_transaction_dictionary_get(void);
+int HIDDEN xbps_transaction_init(struct xbps_handle *);
 
 /**
  * @private