(#11318) Add password management on FreeBSD

This adds the manages_passwords feature to the pw user provider. It is based
on the patch by Andrew Hust that was integrated into FreeBSD puppet port. It
adds tests covering the create, delete and modify processes of the provider.

This integrates a fix for #7500 that was introduced by the original patch.
The existing code takes the first character of each property and uses it as a
flag. However, with pw, the -p flag is for setting the password expiration.
The result is that the password isn't set at create time and that the password
is set to expire. The next run of puppet correctly sets the password but the
expiry is still set. The new code avoids using -p for passwords, and also sets
the password correctly when an account is created.

Reviewed-by: Patrick Carlisle <patrick@puppetlabs.com>