new OTP related options

[mrbaseman]

• print out OTP prompt starting with 'Enter' (which may contain the index of the code in a list) see One-time password prompt problem #427
• add a config file option (and command line parameter) for that
• rebased the --otp-delay option which is still open in Add --otp-delay option #376

[mrbaseman]

We could drop f3b820e again, now that 2fde0a5 offers a more flexible solution, but for testing I have left this commit in my branch
The code changes are at the same places as the ones in #376 so they cause conflicts, so I have cherry-picked the commit from there and merged it manually.
For #427 I feel we should provide a solution and when I had the solution ready and looked at #376 I have noticed that the changes are not only in the same code regions but in fact very similar. So it would be quite unfair to decline the latter one and include a solution for the other one.

[DimitriPapadopoulos]

👍

OK for f3b820e and #376 too. Just add the line break after if for Linux kernel coding style.

[mrbaseman]

sorry for the noise of all those commits. I have squashed the changes into two commits, one for the --otp-prompt and one for --otp-delay and later on squashed a few more updates for the man page into the first one.