Fix UI glitches and do not pre-write the answer to questions with the default anwser

[kit-ty-kate]

Queued on top of #6289
This implements what was described in #6289 (review) with the addition of a UI bugfix:

$ opam pin edit ppxlib
Package ppxlib is not pinned. Edit as a new pinning to version 0.33.1~5.3preview? [y/n] y
Press enter to start "vim" (this can be customised by setting EDITOR or OPAMEDITOR)...[WARNING] The opam file didn't pass validation:
  warning 59: url doesn't contain a checksum
Proceed anyway ('no' will re-edit)? [y/n] 

In this example you can see the Press enter ... line is missing the end of line character. This happens because OpamConsole.pause (used to write this line) uses short_user_input with ~default:"", but this function wasn't made to have its default be anything but a string of length 1 in interactive mode. This is marked evident as the prompt subfunction will move the cursor left then remove one character, which happens to be the issue here.