41CL use of EXCFG, RCLCFG and STOCFG

[Monte Dalrymple]

The MMU registers are stored in page 804, between 80401C and 8040FF. This
entire page has always been reserved for 41CL internal use, as shown in the
Memory Reference. The details of specific locations used are shown in the
manuals for the different 41CL-specific software.

I plan to add six more sets of MMU registers. Three sets will reside at 80411C
through 8041FF and three sets will reside at80421C through 80422FF. All of
these locations are currently unused by 41CL software. This change requires
less than two dozen bytes of code, in the YLIB image.

The only user-visible change will be that the EXCFG, RCLCFG and STOCFG
functions will now accept 0-9 as an argument, where before only 0-3 were
accepted.