Proper response for unknown command

[mickeyl]

The unknown command case is handled wrongly – a real ELM327 (and the behemoth of clones) answers with ?\r\r, not just \r – the question mark is very important.

Reasoning: Said answer is very important, since it is good practice to start the ELM327 probing sequence by sending an invalid command, such as .\r, to ensure a consistent state of the send buffer – hence clients need to rely on a proper answer for unknown command.

[Ircama]

Software changed to manage the invalid answer message in ELM_R_UNKNOWN, which defaults to ?\r\r and can be changed. Documentation updated accordingly.