Add optimizer_switch_disabled to show disabled optimizations

@@optimizer_switch has a lot (currently 36) flags. Most of them should be enabled most of the time (ideally we should have all optimizations enabled).

Currently, it is not easy to check which optimizations are disabled. The idea is to add a @@optimizer_switch_disabled variable which is a view of @@optimizer_switch which lists only the disabled optimizations.

The variable should be read-only. It should not be possible to specify it in my.cnf.

The first user will be the crash reporter. Currently, it prints the value of @@optimizer_switch. It should print @@optimizer_switch_disabled instead.