Better signal handling for obfsproxy

We currently treat SIGINT and SIGTERM equally, by terminating obfsproxy asap.

180-pluggable-transport.txt dictates that:

Proxies should respond to a single INT signal by closing their listener ports and not accepting any new connections, but keeping all connections open, then terminating when connections are all closed. Proxies should respond to a second INT signal by shutting down cleanly.

I think we should conform SIGINT with the 180 spec, and leave SIGTERM as is.