gpg: Avoid superfluous sig check info during import.

* g10/key-check.c (print_info): New.
(key_check_all_keysigs): Print sig checking results only in debug
mode.  Prettify the stats info and suppress them in quiet mode.

This also makes usable stats by prefixing them with the key and the
program name.

• GnuPG-bug-id: T3397
• Signed-off-by: Werner Koch <wk@gnupg.org>