server: more naming convention change
Remove jansson.h inclusion from req.h
Wording: command -> cmd
Server: res module for response building (many stubs now)
Client: use same name for request structs (Req and ReqCmd)
Client: Give up terminal until external state change.

It's now possible to run `cat` and `vim` from `das`! dasc's response handling
is currently very adhoc, though.
Add dasc-dummy, the simpliest dasc implementation for testing