PKINIT ECDH support

PKINIT ECDH support

Add support for elliptic curve key exchange to PKINIT (RFC 5349
section 4).  Extend pkinit_dh_min_bits to allow the string values
"P-256", "P-384", and "P-521", using rough finite-field strength
equivalents to rank them relative to the Oakley Diffie-Hellman groups.

When processing TD-DH-PARAMETERS on the client, only accept the three
Oakley groups or the three supported elliptic curve groups.
Previously we accepted any Diffie-Hellman parameters that passed
EVP_PKEY_param_check()/DH_check() and had equal or better bit strength
to the original proposal.

ticket: 9095 (new)