mtest 0.8.4

• A test program links where no program can be started (the Web). A capability that asks for a
  command (probe::command) ran it through mcpplibs.os's process functions, which every test
  program could reach; a target whose system provides none of them -- wasm32-emscripten, by
  openkal's rule that an absent capability is absent at link -- then linked no test program at all
  (undefined symbol: kal_process_wait, ...), since 0.8.0. The target answers at compile time now:
  asked for a command there, the capability is absent ("this system starts no programs"), and
  nothing of starting one is in the program. Nothing changes where programs are started.