Reintroduce check that CRL Number is in range

Reintroduce check that CRL Number is in range

The CRL number draft clarified what ignoring means and it includes checking
that the CRL number is well-formed again. So do this but continue to ignore
the value for any other purpose. This refactors x509_convert_seqnum() into
a couple of helpers. There's some duplication between crl_check_crl_number()
and crl_parse_crl_number() which could be removed if anyone cares.

tweaks/ok job