gpg: Change some error messages.

* g10/getkey.c (parse_def_secret_key): Change error message.  Replace
log_debug by log_info.
* g10/gpg.c (check_user_ids): Make function static.  Change error
messages.
(main): Change error messages.
* g10/revoke.c (gen_revoke): Ditto.

There are other smaller changes not described above.

This change tries to avoid new error messages so not to increase the
the number of translated strings or break too many existing
translations. It also tries to use existing strings and changes the
quoting to the most common style used in gpg.

Key specifications should in general use double quotes. Other values
should use single quotes. However. sometimes it is not easy to
distinguish between values given on the command line and key
specifications. According to old GNU coding standards diagnostics
should not start capitalized - whether this is a good idea is a
different thing but we used this rules for most strings. However,
strings which are used interactively should be properly capitalized
and end with a dot.

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