sm: Print a better diagnostic for encryption certificate selection.

* sm/certlist.c (gpgsm_add_to_certlist): Add diagnostic and fold two
similar branches.

Without this patch gpgsm printed:

gpgsm[23045]: DBG: chan_6 <- RECIPIENT edward.tester@demo.gnupg.com
gpgsm[23045]: certificate is not usable for encryption
gpgsm[23045]: certificate is good

with this patch a

gpgsm[23045]: looking for another certificate

is inserted into the log.

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