guix-install.sh: Restore compatibility with "yes" invocation.

guix-install.sh: Restore compatibility with "yes" invocation.

Commit 6a2e303d3a had modified prompt_yes_no to only read a single character,
aiming to ease the user experience.  This was, in retrospect, a bad idea, as
it makes user input error more likely and introduces complexity.

This commit reverts to line-oriented input, while preserving the default yes
value so that a user can simply hit 'Enter' at the prompt in place of typing
"yes".

* etc/guix-install.sh (_flush): Delete function.
(prompt_yes_no): Restore line-oriented read.  Remove loop.  Make anything else
than yes means no.  Use Bash features to streamline definition.

Reported-by: Lars-Dominik Braun <lars@6xq.net> and others.