Fix a couple of issues in the interrupt code:

• Replace overly-complicated (and buggy) -a logic with a much simpler version: -a causes all interrupts to be displayed, otherwise only those that have occurred are displayed. This removes the need for any MD code.
• Instead of just making sure intrcnt is large enough, figure out the exact size it needs to be. We derive nintr from this number, and we don't want to risk printing garbage. Note that on sparc64, we end up printing garbage anyway because the names of non-existent interrupts are left uninitialized by the kernel.

Tested on: alpha, i386, sparc64