Make ioSendPacket thread-safe

Author: np511

src/00/link.asm

@@ -161,6 +161,9 @@ _:
 ;;  will end in tears.
 ioSendPacket:
     push bc
+    push af
+    ld a,i
+    di
     push af
         ; io_tx_header_ix is 0xFF when ready to send
         ld a, (io_tx_header_ix)
@@ -196,11 +199,19 @@ _:      ld (io_tx_header), de
         out (PORT_LINK_ASSIST_ENABLE), a
     pop de
     pop hl
+    pop af
+    jp po, _
+    ei
+_:
     pop af
     pop bc
     cp a
     ret
 .abort:
+    pop af
+    jp po, _
+    ei
+_:
     pop af \ ld b, a \ or 1 \ ld a, b
     pop bc \ ret ; Packet in progress, GTFO