meta_payroll_type — US:PTO — distinguish between Earned and Taken

Metadata `payroll-type` will go on `Liabilities:Payable:Vacation` and
`Expenses:Payroll:Salary`, but we need to distinguish as to whether
the Payroll was earned or taken.

Later, `Liabilities:Payable:Vacation` should require a
`payroll-type` metadata of either `US:PTO:Taken` or `US:PTO:Earned`,
and the `CA:` equivalents.