Interrupts: Refuse sub-interrupt handlers where the firmware does

Only the GE and vblank interrupts take user sub-interrupt handlers, and
vblank only in slots 0-15, with some of the rest already held by the
kernel. The errors follow interruptman.prx's checks, and which interrupts
have handlers at all is read back from pspautotests intr/registersub and
intr/releasesub, which now pass.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test.py

@@ -266,6 +266,8 @@ tests_good = [
   "intr/intr",
   "intr/mfic",
   "intr/enablesub",
+  "intr/registersub",
+  "intr/releasesub",
   "intr/suspended",
   "intr/vblank/vblank",
   "io/cwd/cwd",
@@ -570,8 +572,6 @@ tests_next = [
   "gpu/texmtx/uvs",
   "gpu/textures/size",
   "gpu/triangle/triangle",
-  "intr/registersub",
-  "intr/releasesub",
   "intr/waits",
   "sysmem/kernel/heapgrow",
   "io/file/file",