Fix sff panic deadlock, share the evdev probe, assorted review cleanups

ui.go

@@ -449,7 +449,7 @@ func (d *display) drawHeader(phy phyDisplay, hv histView, measuring bool,
 func (d *display) drawColHeaders() {
 	y := d.headerH + 8
 	d.textRightAt(d.small, d.resetX+d.resetW, y, "RESET", uiMuted)
-	hdr := "LAST 90 S"
+	hdr := fmt.Sprintf("LAST %d S", int(histSpan/time.Second))
 	hw := len([]rune(hdr)) * d.small.cellW
 	d.small.draw(d.fb, d.cellsX+(d.cellsW-hw)/2, y-d.small.capTop, hdr, uiMuted)
 	d.textRightAt(d.small, d.nowX+d.nowW, y, "NOW", uiMuted)