Added a graceful way to terminate st-util

[AJ528]

if st-util is requested to close using SIGTERM, do so and exit with status code 0.

[Ant-ON]

With such changes, (void)signum; is not required

[AJ528]

With such changes, (void)signum; is not required

I'll be honest, I don't really know what that line is doing. It comes after exit() so I'm assuming it never gets run. In the spirit of "change as few things as possible" I didn't modify it, but if it serves no purpose I'm happy to remove the line.

[Ant-ON]

@AJ528 this line suppress the warning about signum argument not used.

[AJ528]

alright, I removed (void) signum;