dirmngr: Reject certificate which is not valid into cache.

* dirmngr/certcache.c (put_cert): When PERMANENT, reject the
certificate which is obviously invalid.

With this change, invalid certificates from system won't be registered
into cache. Then, an intermediate certificate which is issued by an
entity certified by such an invalid certificate will be also rejected
with GPG_ERR_INV_CERT_OBJ. With less invalid certificates in cache,
it helps the validate_cert_chain function work better.

• GnuPG-bug-id: T6142
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>