cmCustomCommand: Refactor custom command-specific policy values

Many custom commands are created by CMake itself rather than by the user. These custom commands should always have their policies set to NEW, and user-created custom commands should have their policy values set only from the state snapshot. In addition, we want to genericize the mechanism of recording a policy at the time of custom command creation.

Add a CM_FOR_EACH_CUSTOM_COMMAND_POLICY macro to genericize custom command policies. Use this to define all custom command policies. Make all such policies NEW instead of WARN by default. Remove individual policy modifier methods and add a single method that records relevant values from a cmStateSnapshot. Remove the no longer needed explicit policy settings from synthesized custom commands.