Update Linux Gather User History module

Minor improvements and tidy up of Linux Gather User History module.

* Collect history for additional shells (ash bash csh ksh sh tcsh zsh)
* Collect PostgreSQL history
* Use `blank?` instead of `nil?` to avoid writing empty text files to loot directory.

```/home``` isn't always where home directories live.  This problem existed before this PR and exists in plenty of other modules, but why not use ```~#{u}/whatever``` instead to allow the home directory to resolve client-side?