Use accessors for X509_print_ex().

Print certificate details using accessor functions.

Since X509_CERT_AUX_print is only used in one place and can't
be used by applications (it uses an internal X509_CERT_AUX structure)
this has been removed and replaced by a function X509_aux_print which
takes an X509 pointer instead.

Reviewed-by: Tim Hudson <tjh@openssl.org>