Add: Local user inventory module

The local user inventory module parses and reports local user
information as discovered in /etc/passwd and /etc/shadow. Additional
bundles are provided to augment the Mission Portal Inventory UI with
interesting information such as users with locked accounts, when users
last changed their password, and users that have empty passwords.

Ref: https://dev.cfengine.com/issues/6213
Change: Inventory date of last password change report as epoch

Instead of trying to localize the time format on the client, report in
epoch format so that we can build filters and conditions based on a
standard time format.
Merge pull request #277 from nickanderson/redmine/6213

Add: Local user inventory module