Doesn't reset port on exit

[haraldhh]

There's some problems with resetting the connection when exiting rtl-ais with ^C, it doesn't free up the port and it's left in TIME_WAIT state. I get an 'Error initializing built-in AIS decoder' when trying to start rtl-ais and the program exits. If I wait a while until the TIME_WAIT state of the port is removed I'm able to start rtl-sdr again.