QDict: Introduce new iteration API

QDict: Introduce new iteration API

It's composed of functions qdict_first() and qdict_next(), plus
functions to access QDictEntry values.

This API was suggested by Markus Armbruster <armbru@redhat.com> and
it offers full control over the iteration process.

The usage is simple, the following example prints all keys in 'qdict'
(it's hopefully better than any English description):

   QDict *qdict;
   const QDictEntry *ent;

   [...]

   for (ent = qdict_first(qdict); ent; ent = qdict_next(qdict, ent)) {
        printf("%s ", qdict_entry_key(ent));
    }

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>