Freeswitch upgrade

This new freeswitch version has some substantial changes. sofia-sip is no longer part of main FS tree, but is provided as external package. Some tests fails, so I had to temporary disable the check.

A modified version of spandsp (bumped to version 3.0.0) is also a new dependency. For the latter, Freeswitch does not provide a tagged release, but there's a pending request for that: https://github.com/freeswitch/spandsp/issues/13 Probably I need to put in depends of spandsp3 "!spandsp" to avoid conflicts, but this is a WIP so feel free to comment and give feedbacks.

Thanks. .: Francesco