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>