Implement signal constants on POSIX platforms

[BCSharp]

What has changed:

• The values of existing constants in module signal are changed to reflect the actual values on a given platform (previously the values were always Windows-specific).
• Missing signal constants are added for Unix signals.
• test_signal from the IronPython test suite is modified to pass with CPython on all OS platforms supported by IronPython.
• The implementation of module signal is modified to pass the modified test_signal. This in practice means primarily changes to error handling.

What has not changed:

• The set and values of signal constants on Windows stays the same. For what I can see it is pretty complete.
• Signal handling is the same (this means: on Windows SIGINT and SIGBREAK, and only SIGINT on Unix; no new signals are being handled yet).
• No new (missing) functions are added to signal.

[slozier]

LGTM. Thanks!