sh: improve emacs mode

In emacs mode, force ^R to backware search the history
This behaviour is the default in emacs mode for most of the other shells

Note: Note that this can still be overridden via $EDITRC, ~/.editrc or a
bind command after set -o emacs.

MFC after: 1 week
Approved by: jilles
Reviewed by: jilles, arichardson, pstef
Differential Revision: https://reviews.freebsd.org/D29494