veyron/security/agent/agentd: kill child command when the agent is killed
veyron/security/agent/agentd: kill child command when the agent is killed

If the agent receives a signal like TERM or INT, we want the agent to first kill
its child before exiting.  The change makes agent listen for ShutdownOnSignals
and relay the received signal to the child (so that the child has a chance to
shut down cleanly).

Change-Id: I9301ff092b7b7cdce1d6b7bf655e3ff41811630a

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref