Provide capability to check lock status

[chrahunt]

This can be useful for interactive debugging or not-so-paranoid asserts/logging when a file lock is supposed to be in effect.

Since this isn't provided directly by Python, it would need to be done using ctypes/C/Cython and interact with platform-specific APIs. In order to implement this enhancement we would need to either:

1. Use another library (but currently this library does not have any dependencies)
2. Implement the required functionality in this library (may impact portability, but it could degrade gracefully)