CLEANUP: stconn: rename cs_ep_set_error() to se_fl_set_error()

First it applies to the stream endpoint and not the conn_stream, and
second it only tests and touches the flags so it makes sense to call
it se_fl_ like other functions which only manipulate the flags, as
it's just a special case of flags.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 4a3a701..1e50bed 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -1284,7 +1284,7 @@
 		h2s->errcode = err;
 		if (h2s->st < H2_SS_ERROR)
 			h2s->st = H2_SS_ERROR;
-		cs_ep_set_error(h2s->endp);
+		se_fl_set_error(h2s->endp);
 	}
 }
 
@@ -2756,7 +2756,7 @@
 	h2s_close(h2s);
 
 	if (h2s_sc(h2s)) {
-		cs_ep_set_error(h2s->endp);
+		se_fl_set_error(h2s->endp);
 		h2s_alert(h2s);
 	}
 
@@ -6668,7 +6668,7 @@
 	/* RST are sent similarly to frame acks */
 	if (h2s->st == H2_SS_ERROR || h2s->flags & H2_SF_RST_RCVD) {
 		TRACE_DEVEL("reporting RST/error to the app-layer stream", H2_EV_H2S_SEND|H2_EV_H2S_ERR|H2_EV_STRM_ERR, h2s->h2c->conn, h2s);
-		cs_ep_set_error(h2s->endp);
+		se_fl_set_error(h2s->endp);
 		if (h2s_send_rst_stream(h2s->h2c, h2s) > 0)
 			h2s_close(h2s);
 	}