Add unused fields to some structures in case we need new fields in the 4.1 LTS

Add unused fields to structures where we're likely to want to add a new field at some point during the lifetime of the next Mbed TLS LTS branch.

At least SSL context, for which we wanted a new field in 3.6 (TODO: link), and we just barely managed to find a field that was partially unused that we made dual-use.