Follow ereport/elog styleguide.
Revise worker_checksums_verify_file() interface

Abandon "path parsing" approach. For parity with RelidByRelfilenode(or more
accurately, the SQL-callable wrapper function of same, pg_filenode_relation()),
the function is called with a tablespace, relfilenode and fork number.

Push some of the responsibility of figuring out those details on to the client,
where they probably belong.

worker_checksums_verify_file() still isn't implemented, but at least the call
into the main entry point of verification.c is now sane.