Adds:
1) Simulator side execution of register update activities at an interval
specified in usecs. Rates in excess of 1000Hz should be achievable
for locally connected frontpanel applications.
2) New API for simulators to describe the register state available in
the simulator. If all of a the state which is potentially interesting
to front panel applications is always present in the variables
described by simh REG structures, then frontpanel API access to
that data can be more efficiently provided.
