DBusMessageIter: add `dbus_message_iter_has_signature()`
This function is analogous to `dbus_message_has_signature()`. The
advantage over `dbus_message_iter_get_signature()` + `strcmp()` is
that this needs no dynamic allocations.
I have been long wondering why there was no dbus_message_iter_has_signature()
, so here is a prototype.