csrgen-o: extract X509_NAME creation subroutines
The codes that turns a string into an X509_NAME are big, complex and
way too deeply indented. Extract these to two separate subroutines.
`ldap_dn_to_X509_NAME` tries to turn a string DN into an X509_NAME.
`cn_to_X509_NAME` take a whole string and uses it as the CN in a