X.509: Handle midnight alternative notation in GeneralizedTime

The ASN.1 GeneralizedTime object carries an ISO 8601 format date and time.
The time is permitted to show midnight as 00:00 or 24:00 (the latter being
equivalent of 00:00 of the following day).

The permitted value is checked in x509_decode_time() but the actual
handling is left to mktime64().

Without this patch, certain X.509 certificates will be rejected and could
lead to an unbootable kernel.

Note that with this patch we also permit any 24:mm:ss time and extend this
to UTCTime, which whilst not strictly correct don't permit much leeway in
fiddling date strings.

Reported-by: Rudolf Polzer <rpolzer@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
cc: David Woodhouse <David.Woodhouse@intel.com>
cc: John Stultz <john.stultz@linaro.org>