Backpatch from head: fixes for a threading bug, a problem with setNull involving

Backpatch from head: fixes for a threading bug, a problem with setNull involving
server prepared statements and an improved error message

 Modified Files:
  Tag: REL7_3_STABLE
 	jdbc/org/postgresql/errors.properties
 	jdbc/org/postgresql/core/Encoding.java
 	jdbc/org/postgresql/core/QueryExecutor.java
 	jdbc/org/postgresql/jdbc1/AbstractJdbc1Connection.java
 	jdbc/org/postgresql/jdbc1/AbstractJdbc1DatabaseMetaData.java
 	jdbc/org/postgresql/jdbc1/AbstractJdbc1Statement.java