PKCS#7: Improve and export the X.509 ASN.1 time object decoder

Make the X.509 ASN.1 time object decoder fill in a time64_t rather than a
struct tm to make comparison easier (unfortunately, this makes readable
display less easy) and export it so that it can be used by the PKCS#7 code
too.

Further, tighten up its parsing to reject invalid dates (eg. weird
characters, non-existent hour numbers) and unsupported dates (eg. timezones
other than 'Z' or dates earlier than 1970).

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/crypto/asymmetric_keys/x509_public_key.c b/crypto/asymmetric_keys/x509_public_key.c
index 6b060b2..6d88dd1 100644
--- a/crypto/asymmetric_keys/x509_public_key.c
+++ b/crypto/asymmetric_keys/x509_public_key.c
@@ -302,14 +302,7 @@
 	}
 
 	pr_devel("Cert Key Algo: %s\n", pkey_algo_name[cert->pub->pkey_algo]);
-	pr_devel("Cert Valid From: %04ld-%02d-%02d %02d:%02d:%02d\n",
-		 cert->valid_from.tm_year + 1900, cert->valid_from.tm_mon + 1,
-		 cert->valid_from.tm_mday, cert->valid_from.tm_hour,
-		 cert->valid_from.tm_min,  cert->valid_from.tm_sec);
-	pr_devel("Cert Valid To: %04ld-%02d-%02d %02d:%02d:%02d\n",
-		 cert->valid_to.tm_year + 1900, cert->valid_to.tm_mon + 1,
-		 cert->valid_to.tm_mday, cert->valid_to.tm_hour,
-		 cert->valid_to.tm_min,  cert->valid_to.tm_sec);
+	pr_devel("Cert Valid period: %lld-%lld\n", cert->valid_from, cert->valid_to);
 	pr_devel("Cert Signature: %s + %s\n",
 		 pkey_algo_name[cert->sig.pkey_algo],
 		 hash_algo_name[cert->sig.pkey_hash_algo]);