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.