hbal: change handling of signal

Currently, hbal does a one-two signal handling, where the first signal
causes graceful termination, and the second one an immediate on (either
SIGINT or SIGTERM can be used, interchangeably). However, this poses a
timing problem: if two programs want to send a graceful termination
request, they cannot do that without careful coordination.

To fix this, we change to code to handle the signal separately: SIGINT
(^C) sends graceful termination, while SIGTERM sends immediate
termination. This should allow easier controlling of hbal.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>