x509: use NSS enums and OIDs to identify SAN types
    
    GeneralName parsing currently relies heavily on strings from NSS.
    Make the code hopefully less brittle by identifying GeneralName
    types by NSS enums and, for otherName, the name-type OID also.
    
    Part of: https://fedorahosted.org/freeipa/ticket/6022
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>