Implement some expect-based autopkgtests

Several procServ features can be tested with expect(1) scripts. Implement some expect-based tests to have a minimal test coverage to catch regressions, and also validate some applied patches.

During the development of --killsig-related test, I noticed that another set of proposed patches could be already applied to fix the bug. Thus, I have also included them as part of this series.

After writing this MR, I now think upstreaming a version of these expect-based tests might be a good choice. Let me know if you think the same.