This cast and rename of the `void *arg` to `struct pfdata *pfd`
isn't necessary, we can change the argument type to the actual type,
because the function is called only once and therefore always with
an argument of the `struct pfdata *` type.

People will be able to tell probes to skip certain paths during
content evaluation. They can specify a list of paths by setting the
`OSCAP_PROBE_IGNORE_PATHS` environment variable. The paths in this
list should be separated by a colon.

This cast and rename of the `void *arg` to `struct pfdata *pfd`
isn't necessary, we can change the argument type to the actual type,
because the function is called only once and therefore always with
an argument of the `struct pfdata *` type.

Extend test_skip_paths to test the "skip selected paths" feature
also in the filehash58 probe.

Extend test_skip_paths to test the "skip selected paths" feature
also in the xmlfilecontent probe.