Use SFR access funcs. Support 'dump' command. Add -m (monitor) flag.

Not all SFRs are visible in the unified address space, so the SFR-specific
accessors are required.

The dump command is the same as the various 'd*'
commands, but also supports dumping program memory.

The new -m (monitor) flag watches the command stream between s51 and sdcdb.

Signed-off-by: Keith Packard <keithp@keithp.com>