gpg: Allow direct key generation from card with --full-gen-key.

* g10/call-agent.c (agent_scd_readkey): New.
* g10/keygen.c (ask_key_flags): Factor code out to ..
(ask_key_flags_with_mask): new.
(ask_algo): New mode 14.

Note that this new menu 14 is always displayed. The usage flags can
be changed only in --expert mode, though. Creating and using signing
keys works but decryption does not yet work; we will need to tweak a
couple of other places for that. Tested with a Yubikey's PIV app.

• Signed-off-by: Werner Koch <wk@gnupg.org>