watchdog: reworked to add support for software reset.

support for software reset due to watchdog timeout...
based on watchdog documentation.

modified:   sim/sim_avr.c
modifications to avr_reset and avr_init to support software
reset.

modified:   sim/sim_avr.h
added type avr_run_t...
avr_t.run updated accordingly.

modified:   sim/avr_watchdog.c
largely rewritten to support software reset.

modified:   sim/avr_watchdog.h
data record 'reset_context' added to support software reset

modified:   sim/sim_regbit.h
added: avr_regbit_from_value
for checking flags before commiting register value.
added: avr_regbit_set_array_from_value
reverse operation of avr_regbit_get_array