Werk #4431: Fixed enabling/disabling of notifications/active checks/passive checks

Changing various flags in an ad-hoc (i.e. non-rule-based) manner did not work correctly: Changing a flag to a value different from the configurated one worked, but trying to change it back to its configurated value had no effect. This affected the various Enable/Disable buttons on the host/service details pages plus the corresponding Livestatus commands (ENABLE_HOST_CHECK etc.).

Note that 1.2.8 versions and earlier are were not affected by this bug.

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