upstream: Help OpenSSH's PKCS#11 support kick its meth habit.

upstream: Help OpenSSH's PKCS#11 support kick its meth habit.

The PKCS#11 code in OpenSSH used the libcrypto public key method API
(e.g. the delightfully named RSA_meth_free()) to delegate signing
operations to external keys. This had one advantage - that it was
basically transparent to callers, but also had a big disadvantage -
that we'd manually have to track the method implementations, their
state and their relationships to the underlying PKCS#11 objects.

This rips this out and replaces it with explicit delegation to
PKCS#11 code for externally hosted keys via the ssh-pkcs11-helper
subprocess. This is very similar to how we handle FIDO keys in
OpenSSH (i.e. via ssh-sk-helper). All we need to track now is a
much simpler mapping of public key -> helper subprocess.

Kicking our libcrypto meth dependency also makes it much easier
to support Ed25519 keys in PKCS#11, which will happen in a subsequent
commit.

feedback / ok tb@

OpenBSD-Commit-ID: a5a1eaf57971cf15e0cdc5a513e313541c8a35f0