Option to update Contact URI when sending re-INVITE or UPDATE

[bennylp]

Updated pjsua_call_reinvite() and pjsua_call_update() API to use bitmask flag in the options parameter, to allow application to request that the invite session be updated with the new Contact URI from the account. This is useful in IP address change situation, to update the invite session with the new account Contact after the account Contact is updated (typically with a registration refresh), and send this new Contact in an re-INVITE or UPDATE request.

Two options/flags are created:

• PJSUA_CALL_UNHOLD
• PJSUA_CALL_UPDATE_CONTACT

Note that this update is backward compatible.