Restore some pre-RISC OS 5 ability
A casual comment "May well have broken pre RO5 usage" in DebugTools-1_16 contained much truth; the *IRQDevices command aborted after only a couple of entries due to different node contents. Make a note if this is RISC OS 5 at startup, and select how to read the device node accordingly. To prevent falling off the end of the table also use hardwired IOC and IOMD knowledge to record the device table end, if OS_ReadSysInfo 6 fails to provide.
Tested on RISC OS 3.70 and RISC OS 3.10, no longer aborts with *IRQDevices.