engine_pkcs11: trivial: fix variable declaration style

More tweaks from checkpatch.pl

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
engine_pkcs11: minor: verify that we have a token.

Probably an impossible situation, but checking for the token increases
symmetry at the very least.

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
engine_pkcs11: trivial: rename "tok" to "token".

A minor change, but it fits in better with the surrounding code.

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
engine_pkcs11: minor: remove or reduce scope of variables

As I refactored the code, I accidentally left some variables with
larger scopes than necessary.  In a few cases, some variables were no
longer needed in that scope at all.

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
engine_pkcs11: minor: rename "slot_list" to "slots"

In line with the rest of the code, use the simple plural even if we're
pointing at an array of the things.  (Compare with "keys" and "certs",
instead of "key_list" and "cert_list".)

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
engine_pkcs11: minor: use "slot_id" consistently.

Previously, there were 3-4 different identifiers that referred to the
same basic concept.  Make the concept clear through naming.

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
engine_pkcs11: minor: discard "selected_..." prefix.

Especially after refactoring, the functions for selecting a requested
certificate or key are small enough that we can use shorter
identifiers.  Getting rid of the leading "selected_..." also makes the
code more consistent (e.g., "slot" vs. "found_slot"
vs. "selected_slot").

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
engine_pkcs11: trivial: rename "isPrivate" to "is_private"

The engine itself prefers all-lowercase + underscores for names.
libp11 uses camelcase.  Ugly, but let's keep each layer consistent.

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
engine_pkcs11: remove unused 'pkcs11_rsa_finish' function

I'm not sure, but I suspect that this was an earlier attempt to
properly clean up when the key was deleted.  However, at the moment,
it's not compiled in, so we delete it.

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>
engine_pkcs11: consolidate engine methods, add documentation.

All of the "external" documentation is now in the header file.
Internal-only items now have regular comments, not docbook style.

Signed-off-by: Anthony Foiani <anthony.foiani@gmail.com>