channel_variable: sdp_m_per_ptime
Adds a new m= line for each distinct ptime in codec list.

When this variable is not set:
	When mixing codecs with various ptime in a codec list, they will now be allowed to co-exist in the sdp but it will send no ptime attr.
		This means the ptime preferences on the offer will be ignored when mixing codecs with various ptimes.
	When receiving a codec list with no ptime attr, the ptime will be chosen from local preference instead of assuming 20ms
		This means if offer contains PCMU with not ptime and FS has PCMU@40i

Dynamic payloads will now start at 98 and increment per additional dynamic codec per call.
	So now you can add CELT@32000h,CELT@48000h and each one will be auto-assigned a dynamic paylaod type.