mysqlcompat - mysqlcompat: Implement make_set().

Log Message:
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Implement make_set().
Note: we don't support NULL arguments to make_set().
This is because we use arrays underneath make_set() to ease implementation.
We wont have NULL array element support until 8.2

Modified Files:
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mysqlcompat:
README (r1.7 -> r1.8)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/mysqlcompat/mysqlcompat/README.diff?r1=1.7&r2=1.8)
string.sql (r1.9 -> r1.10)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/mysqlcompat/mysqlcompat/string.sql.diff?r1=1.9&r2=1.10)
uninstall.sql (r1.7 -> r1.8)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/mysqlcompat/mysqlcompat/uninstall.sql.diff?r1=1.7&r2=1.8)