tpm2: add handling for elliptic curve keys
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tpm2: add handling for elliptic curve keys

* agent/divert-tpm2.c: Support ECC.

This adds handling for the way gnupg does elliptic keys, namely ECDSA
for signatures and using ECDH with an ephemeral key to generate an
encrypted message. The main problem is that the TPM2 usually has a
very small list of built in curves and it won't handle any others.
Thanks to TCG mandates, all TPM2 systems in the USA should come with
NIST P-256, but do not come with the Bernstien curve 25519, so the
only way to use the TPM2 to protect an elliptic curve key is first to
create it with a compatible algorithm.