Check ownership too?

Hey, I like this library, it'd be good if it could also check whether the current user is likely to be able to execute the file, not just whether the file's permission bits allow it. :)

Start by reading the library's existing executable-file permission check and determine how it currently interprets permission bits. Define how the current user's ability to execute the file should be evaluated across supported systems, then verify that the result distinguishes permission bits from effective ownership and execution access.

Written by the indexing model from the issue text.