include: add nm-dbus-compat.h

include: add nm-dbus-compat.h

Add a file containing the defines like DBUS_INTERFACE_DBUS from
dbus-shared.h, and use it from the gdbus-using files.

Also, convert a bunch of other places that were previously hardcoding
the string values to use the defines instead, and fix the ifcfg-rh
plugin to properly namespace its own D-Bus-related defines.