Add SIGINT handler for stlink cleanup

SIGINT causes st-util to immediately exit, without closing the open
stlink.  This leaves devices (at least the F4 Discovery) in a state
where they are unable to reset.  st-util could still connect and control
them, but a power cycle was required before they could reset on their
own.

A signal handler is added for SIGINT, which performs cleanup and closing
of the open stlink device, allowing it to function normally on
disconnect.
Merge pull request #135 from prattmic/signal_cleanup

Add SIGINT handler for stlink cleanup