Add proper termination of link with XSB with forced termination

[Joachim Jansen]

Pressing ctrl+c during any instance when the XSB process has been initialised will lead to a core dump. Ideally, IDP should just quit "quietly" without printing out messages like "stack smashing detected" that can easily be interpreted as something going horribly wrong.

The attached file contains one such core dump.

[Joachim Jansen]

Suggested fixing strategy:

Maintain a state of whether XSB has been initialised

Fix runidp.cpp so that void SIGINT_handler(int) also calls XSB's close() before doing exit(1)