First implementation of Reliable Data Transfer feature in Ring.
This implementation is a draft, comes with a Python script
tools/dringctrl/sendfile.py to play with and doesn't
implement all the API as described into "datatransfer: API proposal"
This version uses TLS over TCP-TURN sockets to encrypt data.
Transfers require a TURN server with TCP peer connections
as described by the RFC-6062.
Testing: Currently only sendFile API is implemented and data are
saved into a temporary file saved in "/tmp/ring_XXXXXX",
where XXXXXX are replace by mkstemp() command.
Reviewed-by: Olivier Soldano <firstname.lastname@example.org>