add dtmf relay config sync

Wenpeng Songrequested to merge
RFC4733 into devel

Add a new policy dtmf_mode to the ubus request.

libvoice-brcm:https://dev.iopsys.eu/voice/libvoice-broadcom/-/merge_requests/20
libvoice-d2: https://dev.iopsys.eu/voice/libvoice-d2/-/merge_requests/15

Edited by Wenpeng Song

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