mlmmj-process: address TODO to free parsed headers

mlmmj-process: address TODO to free parsed headers

Add free_parsed_hdrs() helper to free readhdrs and all strlist
variables (fromemails, toemails, ccemails, etc.) before exit.

This is a cosmetic fix since mlmmj-process is a short-lived
executable that exits immediately after processing, so the OS
reclaims all memory anyway. However, it improves code hygiene
and removes the XXX comment.

Fixes: 4059b78 ("do_all_the_voodo_here introducton plus a massive cleanup")