linkchecker: Organize state into a struct, and add report.

Moves all the state into a struct so it doesn't need to be passed around
as much.

Also adds a report showing how long it took and what it found.

This includes a minor change: a failure to load a file is now an error,
instead of being ignored. This should only happen if there is a
permission error or some other shenanigans going on.

Author: ehuss