gpg: Merge duplicated code for get_user_id et al.
* g10/getkey.c (get_user_id_string): Add args mode and r_LEN. (get_user_id_string_native): Add new args. (get_long_user_id_string, get_user_id): Rewrite using get_user_id_string.
- Signed-off-by: Werner Koch <wk@gnupg.org>