Reserve opcode 0 for "a2jGetMapping()"

If function mapping is disabled then return an error.
Additionally, use a duplicate pointer to "a2jEcho()" in
the jump table in this case instead. This constitutes
another security measure.
Fix some minor issues

Namely some typos and a wrong type declaration.
Change to Interrupt Pipes
Refine j2aMany() and use it for A2J properties