Make TS structures opaque.

Most of the accessors existed and were already used so it was easy.
TS_VERIFY_CTX didn't have accessors/settors so I added the simple and
obvious ones, and changed the app to use them.  Also, within crypto/ts,
replaced the functions with direct access to the structure members
since we generally aren't opaque within a directory.

Also fix RT3901.

Reviewed-by: Tim Hudson <tjh@openssl.org>