Excel export does not work with >31 character worksheet names

In latin 1 mode, the PEAR library throws an error when a sheet name is longer than 31 chars.

In Unicode mode, the PEAR libarary thinks that longer sheet names will work, but we are getting errors when we try to open those files in Excel.

I propose to truncate all worksheet names to 31 chars max in excellib.

I am about to check this change into OU Moodle. please review and decide if you want it in 1.9 and or HEAD.