sh: improve emacs mode

in emacs mode ^W should delete the previous word by default
Note that upstreaming this change directly into libedit is in process.

Reported by:	manu
Reviewed by:	jills, pstef, manu
Differential Revision:	https://reviews.freebsd.org/D29493

bin/sh/histedit.c

@@ -204,6 +204,7 @@ histedit(void)
 			else if (Eflag) {
 				el_set(el, EL_EDITOR, "emacs");
 				el_set(el, EL_BIND, "^R", "em-inc-search-prev", NULL);
+				el_set(el, EL_BIND, "^W", "ed-delete-prev-word", NULL);
 			}
 			el_set(el, EL_BIND, "^I", "sh-complete", NULL);
 			el_source(el, NULL);