rtems: Generate <rtems/rtems/intr.h>

Change license to BSD-2-Clause according to file histories and
documentation re-licensing agreement.

Update #3899.
Update #3993.

cpukit/include/rtems/irq-extension.h

@@ -29,7 +29,7 @@ extern "C" {
 /**
  * @defgroup rtems_interrupt_extension Interrupt Manager Extension
  *
- * @ingroup ClassicINTR
+ * @ingroup RTEMSAPIClassicIntr
  *
  * In addition to the Classic API interrupt handler with a handle are
  * supported.  You can also install multiple shared handler for one interrupt