Provide ABORT-CALLBACK and EXIT-EVENT-LOOP restarts for callbacks

Provide ABORT-CALLBACK and EXIT-EVENT-LOOP restarts for event loop callbacks. Make one of them the default for sldb when swank is active (bound to 'q' in sldb buffer) depending on whether SLIME REPL integration is used. This helps against unwanted event loop termination when using as-repl. Another problem was that (if I understand correctly) unwinding the stack through uv_run() call has unspecified consequences, so it's better to stop event loop via uv_stop() if event loop termination is desired upon an unhandled condition.