Enable basic command line editing in amd64 installer shells

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

There is no point in putting a second sysinst binary into the install
image's root filesystem.

branches: 1.1.2;
Restore having a BIOS-only amd64 USB image.

Several people reported having hardware that struggles booting the mix
EFI+BIOS image, and it's better to offer them a working image.

Keeping the EFI image as having the obvious name, this is going to become
more likely to work by default as newer hardware may not support BIOS boot.

(XXX pullup-9)

There is no point in putting a second sysinst binary into the install
image's root filesystem.

branches: 1.1.2;
Restore having a BIOS-only amd64 USB image.

Several people reported having hardware that struggles booting the mix
EFI+BIOS image, and it's better to offer them a working image.

Keeping the EFI image as having the obvious name, this is going to become
more likely to work by default as newer hardware may not support BIOS boot.

(XXX pullup-9)