[dwm] save cpu cycles - bar status and wrapper scripts

Hello,

I have a wrapper script for dwm that checks a bunch of devices running
on my laptop. I was thinking that a lot of times I don't even have the
dwm bar up (i tend to hide it often for full screen work) and so why
should the wrapper script constantly poll for status bar info if I
can't see it. Long story short, I made a small change to write out a
file to /tmp whenever the status bar is visible. Any thought if this
is the right approach? Is there a less barbaric way to alert my dwm
wrapper script that the bar is hidden?

Diff is attached. I also added a config param to specify the filepath
for the file that gets written.

Cheers,
Alex