even if myisam-recover is OFF

The problem was that a corrupted MyISAM table was auto repaired
even if the myisam_recover_options server variable (or the 
myisam_recover option) was set to OFF.

The reason was that the auto_repair() function, which is supposed
to say if auto repair is to be used, did not use the server variable
setting correctly. This bug was a regression introduced by WL#4738.

This patch fixes the problem by making sure auto_repair() returns
FALSE if myisam_recover_options is set to OFF.

Test case added to myisam.test.