Consolidate BSD shadow support into single module

Since OpenBSD works the same way as NetBSD, instead of writing a third
module this commit consolidates BSD shadow operations into a single
module.
Add OpenBSD to the list of OSes which support password hashes
docstring tweak
Add note to docstring reflecting OpenBSD shadow support
Merge pull request #5765 from terminalmage/bsd_shadow

Add shadow support for OpenBSD