If Shaman is used to submit the job files, store the job's checkout ID
(i.e. the path relative to the checkout root) in the database. This will
make it possible in the future to remove the Shaman checkout along with
the job itself.
Shaman was made on Linux, using the `path` module, which only supports
forward slashes. This mostly replaces `path` with `path/filepath` to use
native paths and increase Windows compatibility.
The work isn't done yet, though.
This is a better way to check errors, both semantically (no usually nil is
bad, but nil error is good, so this avoids a mental negation) and for
the reporting by the assert package.
