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.

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

(cherry picked from commit 660045fb5360d328da199ae78c89d5cc15b6d1ca)