Add SIGTERM handling

[shunyy]

Current generated(goa example) code states that it supports SIGINT and SIGTERM for graceful shutdown.

// Setup interrupt handler. This optional step configures the process so
// that SIGINT and SIGTERM signals cause the services to stop gracefully.

But it actually support only SIGINT, not SIGTERM.
I think it is better to support SIGTERM for better support of kubernetes or ECS (and so on...) container lifecycle.

[raphael]

Makes sense, thank you. Please back-port to v2 as well, thanks again!