dbus-message: bring back the original layout of DBusMessageIter

dbus-message: bring back the original layout of DBusMessageIter

The layout introduced with GVariant was kind of mimicking
the real structure, as opposed to the original layout, which
was taking into account only total sizes.

Dbus-1.10.8 introduced some static assertions for the properties of
DBusMessageIter. These are incompatible with the "GVariant" layout.

Moreover, they show that the "GVariant" layout could be prone to
the copying, that could omit some fields or parts of fields, due
to its not "packed" structure.

This commit brings back the original layout, with modifications
suited for GVariant-sized DBusMessageRealIter.

As an additional feature - this should fix compilation for 64-bit
architectures.

Change-Id: Ie1cac3748d408c22acf177ede5fb9e4e29818c9b