core: rework ask_for_confirmation()

core: rework ask_for_confirmation()

Now the reponses are handled by ask_for_confirmation() as well as the report of
any errors occuring during the process of retrieving the confirmation response.

One benefit of this is that there's no need to open/close the console one more
time when reporting error/status messages.

The caller now just needs to care about the return values whose meanings are:

 - don't execute and pretend that the command failed
 - don't execute and pretend that the command succeeed
 - positive answer, execute the command

Also some slight code reorganization and introduce write_confirm_error() and
write_confirm_error_fd(). write_confim_message becomes unneeded.