Possible compatibility problems with earlier versions marked with '*'.
Added extended attributes for retrieval module (data1), so that data1
-records carry d1_readXML/SGML attributes.
+records carry d1_readXML/SGML attributes. d1_read also supports
+null-data rules ( <tag/>)
On UNIX when POSIX threads are available, a separate library libyazthread
is created. Programmers should link with that library and the yaz library