When a user selects "exist installer" or hits ^Z in sysinst, the
calling install.sh runs a shell, plus shows a message indicating
they can type "exit" to return to the installer.
Add -E to the arguments to this shell, to enable command line editing,
(primarily for arrow keys, but basic emacs sequences will also work)
At this point this is only on amd64, as this is the port which is
least likely to be installing on particularly esoteric terminal
hardware, and is also likely to have the highest proportion of less
experienced users
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