PR11105: robustify stap-server

PR11105: robustify stap-server

* main.cxx (main): Always downgrade client-provided -p5 to -p4.
* stap-client (unpack_response): Sanitize stdout due to same.
* stap-server-connect.c: Eliminate a bunch of globals.
  (handle_connection): Make things locals instead.  Base tmp files
  on $TMPDIR.
  (spawn_and_wait): New helper function.
  (handleRequest): New monster function to inline rest of old
  stap-server-request.