Provide dfrotz `f_setup.command_name` with an initial buffer

Provide dfrotz `f_setup.command_name` with an initial buffer

Without this initialization `f_setup.command_name` is NULL, but
e.g. `replay_open()` assumes that it points to a valid buffer, and produces a
segfault if it does not.  In particular, this causes `dfrotz` to segfault when
attempting to replay a command playback file.