Add fax/t38 support

[ctradu]

The guys at https://code.osso.nl/projects/sipp have a nice fork of sipp 3.2 that adds support for t38 fax in sipp.

A new directive send_pcap_image is added and the support for the pcap files containing fax images.

With the permission of the authors, we could add this feature in sipp.

[rkday]

I agree that this would be useful - I'll get in touch with the osso.nl team about merging their changes back.

[wdoekes]

I have the changes ready, but they were made on top of #57 which is not merged yet.

[kumbasar]

any updates about the merge?

[wdoekes]

Not much to say, unfortunately. #57 cannot be merged because it regresses existing functionality. It needs an overhaul. And no one has gotten around to doing that yet.