entry: use IMR to try detect GPF [fixes #237]

Using ISR is unsafe because programs can legally chain to the prev
inthandler after EOI. We can rely on the fact that IRQ5 is masked
in IMR by default. If this is the case, then its most certainly a
GPF, so we can terminate the process. But if its not masked, always
pass to prev inthandler.

kernel/entry.asm

@@ -221,12 +221,10 @@ rst0d:
                 retf
 hdlr0d:
                 push ax
-                mov al, 0Bh			; request In-Service Register (ISR)
-                out 20h, al			; from primary PIC
-                in al, 20h			; read the ISR
-                test al, 10_0000b		; IRQ #5 occurred ?
+                in al, 21h			; read the IMR
+                test al, 10_0000b		; IRQ #5 masked ?
                 pop ax
-                jnz @2				; yes, (likely) not a GPF
+                jz @2				; no, (likely) not a GPF
                 mov si,gpf_message
                 jmp zero_message_loop
 @2: