[ASTERISK-176] chan_pjsip: Add "none" DTMF method

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https://github.com/search?q=repo%3Aasterisk%2Fasterisk%20AST_RTP_DTMF_MODE_INBAND&type=code

We need to add a "hybrid" sort of DTMF method called "none". None is similar to inband, except no DSP is set up and no DTMF detection is actually performed. In other words, it's a transparent connection where Asterisk itself does no DTMF processing or detection, and it is assumed that inband DTMF is used, but the user agents on both ends of the connection do the DSP to process the DTMF, or there is no DTMF detection desired at all.

This is needed to make MF signaling work reliably when using PJSIP. Inband DTMF needs to be negotiated in the SDP so that the carrier doesn't try to do any DTMF conversions and screw up the signaling, and we ourselves also need to NOT do any DTMF detection to avoid the same fate ourselves. So, make the other guy think that we'll do the DSP, but don't actually do anything. Hence, "none".

This is (at least one place) where we don't set up the DSP: https://github.com/asterisk/asterisk/blob/308c547a216654af5ab6b6b3eb2201b566a64cdd/channels/pjsip/dialplan_functions.c#L1153

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