Fix interface return type and remove unused dummy argument

[stefabat]

This PR fixes two problems with the Fortran API that I encountered while integrating TREXIO into CP2K:

1. The interface to function trexio_info() was returning an integer rather than a c_int32_t. To fix it @scemama and I used the same trick used for function trexio_open.
2. The subroutine trexio_strarray2str had an unused dummy argument, max_str_len, which we removed from the signature and for the relevant subroutine calls.

[q-posev]

Thanks @stefabat! Would you like me to review the PR or @scemama will take care of it?

[scemama]

We did it together, so I would prefer if you could give a quick look ;-)

[q-posev]

Looks good to me!