Package gambit-signal: Information

- Use Gambit's interrupts to pass on the received signals.
- Raise the exception interrupting the recipient thread.
- Exit with status of 1 when the test fails.

- Add the proc to access the signal number field in a signal
  exception.

- Bundle sources in plain tar.
- Add the test program.
- Add getpid and getppid wrappers.
- Add the Makefile and use it.
- Refactor the sources, update the licensing information.