* Centralize all Cadillac interface defs into defs.h.

* Centralize all Cadillac interface defs into defs.h.
* Use prototypes for all interface routines (fix bugs found as a result of
  doing so).
* Make full_filename more robust in the face of null dirname/filenames.
* Get pty at startup instead of at child creation time.  Put GDB descriptors
  0, 1 and 2 on it so that we can see the output of shell commands.
* Get rid of prompt(), use print_prompt() instead.  Fixes loads of stupid
  prompting problems.