Revitalise System.Posix.DynamicLinker.Module

1. This module has virtually no documentation: there is neither a high-level overview of the API, nor any haddocks for individual functions.
2. `withModule` function is partial and will throw an error `Prelude.tail: empty list` when `mdir` is `Just ""`. This must be dealt with one way or another.

https://github.com/haskell/unix/blob/b0856460e7d5ef05ab2ed851498d9f4586fda29a/System/Posix/DynamicLinker/Module.hsc#L96-L106

Start with System/Posix/DynamicLinker/Module.hsc, especially the withModule implementation around lines 96-106. Review the module's exported API and document its purpose and individual functions, then address the Just "" case and verify that withModule no longer fails with Prelude.tail: empty list.

Written by the indexing model from the issue text.