gpg: New option --forbid-gen-key.

* g10/gpg.c (oForbidGenKey, opts): New option.
(mopt): New local struct
(gen_key_forbidden): New.
(main): Set and handle the option.

In large system installation it is sometimes useful to make it a bit
harder for users to generate their own keys. An example is a policy
to not use on-disk keys.