(#11046) Add support for user expiry in pw user provider

Add support for setting an expiry date for a user in the pw user
provider. FreeBSD uses the format DD-MM-YYYY rather than Puppet's
YYYY-MM-DD. Tests added to confirm the value is correctly swapped
around.

Also added custom accessor method to take the unix timestamp given
by the operating system to a Puppet-style YYYY-MM-DD. This stops
Puppet from repeatedly trying to set the expiry date if it's already
correct.