menu: an incoming call should not change the current call

[cspiel1]

Additionally, the ringtone is only started for the first incoming call.
The ringtone is not stopped if another call is incoming.

If there are multiple incoming calls, then the first incoming call is answered with command accept by default.

Current behavior in main: The last incoming call is answered.

[cspiel1]

@alfredh you wrote once into call.c

/* NOTE: The new call must always be added to the tail of list,
 *       which indicates the current call.
 */
 list_append(lst, &call->le, call);

Is this still wanted? Or should we re-define the head of the list as current call?
As a workaround I added to menu.c in this PR:

menu_selcall(menu.curcall);

@juha-h If there are multiple incoming calls at the Android baresip app, which one is shown on the screen? The first one or the last one?

[juha-h]

My Android app does not currently support multiple calls except during call transfer. If a call comes in and there already exists a call, the app responds with 486 Busy.

[alfredh]

the tail is used to indicate current call.

This logic could be changed or improved, if you think it is faulty ...

[cspiel1]

the tail is used to indicate current call.

This logic could be changed or improved, if you think it is faulty ...

Okay, I will think about this.