ACI - ahoi configuration interface

interface:
- command (CMD) and responses (RSP)
- truth table
CMD | RSP | logic
 T     T    setter: CMD produces response
 T     F    setter: CMD is echoed (with effective values)
 F     T    getter: empty CMD with response
 F     F    not supported*

* = how about RESET and BOOTLOADER?

types for CMD and RSP are always equal (and go by the name ACI_TYPE_xxx), where xxx is the name of the command 


Questions:
- what happens in case of an unknown CMD (return something)
- use 0 address as serial host? (this would likely clarify a bunch of things)


Not implemented yet:
- xxxstat_reset
- allstat (should be done by modem.py/client)


FIXME
- switch() case workaround in aci.c
- freqsetup