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
Contribution info for OHAI-561.
Merge pull request #303 from swalberg/getpwent_duplicates

[#OHAI-561] Ignore users if we've already seen them
Merge pull request #334 from opscode/sersut/CC-99

Contribution info for OHAI-561.
OHAI-573: Only claim to be an LXC Host if we're pretty sure that we are
OHAI-573: Release notes
Fix warning message about constants already defined
OHAI-572: Release notes
Add mdadm plugin / specs

Grab data on mdadm raid arrays in ohai
Use equals not be to deal with types
Merge branch 'OHAI-578'
Detect additional CPU strings that are KVM
Switch back to relative path and add specs
Consolidate spec
Merge branch 'OHAI-575'
Present all CPU flags on FreeBSD

Don’t toss the first set when we find a few more
Fix execution and add a spec
Add addition Freebsd CPU unit tests
Merge branch 'OHAI-568'
OHAI-568, OHAI-575, OHAI-578: Release notes