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>