SSL.File: Changed backend handling.

The backend to use (real/local) now strictly depends on blocking/
nonblocking mode. This means that creation of the local backend
can be (and has been) moved to set_blocking{,_keep_callbacks}().

This reduces the number of short-lived backends used by SSL in a
typical nonblocking backend-driven application down to ~ zero.
SSL.File: Removed some now ignored arguments.

The two last arguments to RUN_MAYBE_BLOCKING() are no longer used,
so there's no point in having them.