Chris made me aware that ASCII code 0 actually is a valid character (usually entered as Ctrl-Space or Ctrl-@) and that it is actually used (i.e. by EMACS).
The Apple II allows to natively enter the ASCII code 0 via Ctrl-@. However so far get_key_if_available returned 0 in accumulator to signal that no key was pressed. In order to allow the Apple II get_key_if_available to return the ASCII code 0 in the accumulator I changed it to use the carry flag to signal that no key was pressed.
Because get_key_if_available needs of course to behave the same on all targets I changed the other implementations to use the carry flag too.
Unfortunately I don't know enough about input capabilities of the C64 to decide on how to best get Telnet65 to send ASCII code 0 there.
It's imho in general a pretty unfortunate design to have IP65 check the keyboard during blocking operations. Rather it should call back into the application and have that decide what type of user abort it wants to offer.
Anyhow I don't want to change all that - at least not now. Therefore I just added the option to influence which key is considered the abort key - and provide a key value that isn't actually used and as such disables the abort check.
