gpg: Remove restrictions for the name part of a user-id.

* g10/keygen.c (ask_user_id): Allow for the name to start with a
digit.  Allow names shorter than 5.

The reason for this change is that we don't enforce these constraints
in the --quick-gen-key interface. I added the constraints right in the
beginning of gnupg to make sure that we have a uniform style for
user-ids. However, this is all problematic with non-Latin names
and we prefer to use mail addresses anyway.