gpg: Use algorithm id 22 for EdDSA.

* common/openpgpdefs.h (PUBKEY_ALGO_EDDSA): Change to 22.
* g10/keygen.c (ask_curve): Reword the Curve25519 warning note.

In the hope that the IETF will eventually assign 22 for EdDSA using
the draft-koch-eddsa-for-openpgp-01 specs we start using this number.