Let curses do the dirty work for flash

Use the terminfo delay syntax ($<x>) in our flash capability to avoid
hardcoding a fixed delay in redraw() when called from tsetmode() with
DECSCNM.
We need to turn on the npc capability so that delays are made with
xon/xoff instead of padding characters.

st.info

@@ -32,7 +32,7 @@ st| simpleterm,
 	el=\E[K,
 	el1=\E[1K,
 	enacs=\E)0,
-	flash=\E[?5h\E[?5l,
+	flash=\E[?5h$<80/>\E[?5l,
 	fsl=^G,
 	home=\E[H,
 	hpa=\E[%i%p1%dG,
@@ -150,6 +150,7 @@ st| simpleterm,
 	mir,
 	msgr,
 	ncv#3,
+	npc,
 	op=\E[39;49m,
 	pairs#64,
 	mc0=\E[i,