[MINOR] frontend: rely on the frontend and not the backend for INDEPSTR

Till now, the frontend relied on the backend's options for INDEPSTR,
while at the time of accept, the frontend and backend are the same.
So we now use the frontend's pointer instead of the backend and we
don't have any dependency on the backend anymore in the frontend's
accept code.
diff --git a/src/frontend.c b/src/frontend.c
index e2f24ad..fc40929 100644
--- a/src/frontend.c
+++ b/src/frontend.c
@@ -152,7 +152,7 @@
 	s->si[1].fd = -1; /* just to help with debugging */
 
 	s->si[1].flags = SI_FL_NONE;
-	if (likely(s->be->options2 & PR_O2_INDEPSTR))
+	if (likely(s->fe->options2 & PR_O2_INDEPSTR))
 		s->si[1].flags |= SI_FL_INDEP_STR;
 
 	s->srv = s->prev_srv = s->srv_conn = NULL;