[build] cleanup conditionals to build SCTP and other protocols

Unsupported protocols just need to return NULL on get_PROTO_transport
and contain all protocol conditionals within the transport_PROTO.c.

There is still one conditional in RX/TX thread that needs cleaning
but it's a rather complex operation and leave it in for now.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>