Windows portability: There's no SIGHUP on Windows
Windows portability: There's no SIGHUP on Windows

This fixes "AttributeError: 'module' object has no attribute 'SIGHUP'".

Furthermore, the Python documentation for `signal.signal()` [1] says:

> On Windows, signal() can only be called with SIGABRT, SIGFPE, SIGILL,
> SIGINT, SIGSEGV, or SIGTERM. A ValueError will be raised in any other
> case.

[1]: https://docs.python.org/2.7/library/signal.html#signal.signal

stb-tester/stbt-rig