tal.c: Patch some small bugs

tal.c: Patch some small bugs

- Code wasn't validating null result on strdup
- If a validation thread was interrupted,
  `perform_standalone_validation()` was reading an uninitalized
  exit status.

More or less as a side effect, I also merged the structures
`pthread_param` and `thread`, because their usage was similar
and shared ~50% of their members.

`do_file_validation()` is no longer responsible for freeing its
generic argument.