hbal: implement user-friendly termination requests

Currently, hbal will abort immediately when requested (^C, or SIGINT,
etc.). This is not nice, since then the already started jobs need to be
tracked manually.

This patch adds a signal handler for SIGINT and SIGTERM, which will, the
first time, simply record the shutdown request (and hbal will then exit
once all jobs in the current jobset finish), and at the second request,
will cause an immediate exit.