Ignore users if we've already seen them

For environments with duplicate users in different password databases,
such as files and ldap, ohai was using the _last_ entry returned by
`getpwent` but the system uses the first.

Incorporate feedback from @sersut
Merge pull request #303 from swalberg/getpwent_duplicates

[#OHAI-561] Ignore users if we've already seen them