gpg: Add a communication object to the key generation code.
* g10/keygen.c (struct common_gen_cb_parm_s): New. (common_gen): Add args common_gen_cb and common_gen_cb_parm. Adjust all callers. (do_generate_keypair): Clarify the code by using a better var name.
We may eventually also replace the long arg list with that object.
The immediate reason for this change is the followup commit.