A field 'mpty' is added to ofono_call so that we can know
whether the call belongs to multiparty call or not. According
to 27.007 7.18, it is a defined return value of AT+CLCC.
This utility will be used by HFP voicecall driver to compare
two calls. In some hardware, the call index may be shift when
a call transitions from waiting to incoming state (e.g. all other
held and active calls are released)