New callback to notify application when registration or unregistration has been initiated (thanks Tony Jago Million for the patch)

[bennylp]

A new callback is added to pjsua_callback to allow application to be notified when the library has started or stopped registration:

    /**
     * Notify application when registration or unregistration has been
     * initiated. Note that this only notifies the initial registration
     * and unregistration. Once registration session is active, subsequent
     * refresh will not cause this callback to be called.
     *
     * @param acc_id	    The account ID.
     * @param renew	    Non-zero for registration and zero for
     * 			    unregistration.
     */
    void (*on_reg_started)(pjsua_acc_id acc_id, pj_bool_t renew);

[bennylp]

(In [3594]) Closed #1313 (Account option to disable registration when account is added) and closed #1314 (New callback to notify application when registration or unregistration has been initiated). Thanks Tony Jago Million for the patch