gpgsm: select unexpired certificates skipping expired ones

* sm/certchain.c (check_validity_period_cm):  Make function global.
* sm/certlist.c (gpgsm_add_to_certlist): If an expired certificate is
found, continue looking for another one.

This enables the user to select a certificate by subject, and keep
old expired certificates in the store in case he wishes to decrypt
or verify an old file. This makes renewal of certificate smoother.

Due to a broken patch I had to massage the patch and while doing this
also fixed the indentation and moved a declaration to the begin of a
block. - wk@gnupg.org