Improve clarity of product configuration so that names more explicitly indicate what they do:
* CONFIG_POLL_NONE (because the user-visible consequence is that polling is no longer supported; was CONFIG_PROTECTION_NONE).
* DISABLE_LOCKS (was THREAD_SINGLE).
* DISABLE_SHIELD (was THREAD_SINGLE && PROTECTION_NONE)
* DISABLE_REMEMBERED_SET (was PROTECTION_NONE)
When the shield is disabled, ArenaLeave asserts that there are no busy traces, and ArenaPoll is a no-op.
By having functions implemented using the corresponding macro, we can avoid duplicated code, and avoid testing DISABLE_SHIELD in global.c.
Remove all remaining references to MPS_PROD_EPCORE.
We can't run the test cases yet with CONFIG_POLL_NONE.