cobalt/intr: introduce IRQ disabled state
With the IRQ line enable/disable operations restricted to the root domain, interrupt handlers may not invoke xnintr_disable() directly to shut down the interrupt line. To support this requirement, we add an explicit disabled state to the interrupt descriptor, which the handler may turn on by returning the new XN_IRQ_DISABLE operation status. In response, the core disables the descriptor, shutting down the interrupt line in the same move instead of unmasking it on return from the interrupt context. XN_IRQ_DISABLE is exposed by RTDM as RTDM_IRQ_DISABLE. As a consequence of this change, XN_ISR_NOENABLE was dropped as we have no in-tree client which need such semantics anymore. At this chance, various naming and documentation fixes have been applied.