External semaphore to only invoke sleepnow once

Credit goes to @benbosman

Fixes #2

I've slightly modified original script, specifically:
- removed `sleep 10` altogether, since it's not reliable, and adds unpredictable side effects;
- removed parenthesis and ampersand around `pmset sleepnow` so that the script only exits when this command does;
- used a temporary file as semaphore. It might attempt to put the sleeping machine to sleep again because of the change in semaphore file name in between script runs (as $TMPDIR changes), but it won't be doing that in two second intervals causing failures.
- some cosmetics

README.md

@@ -61,4 +61,6 @@ It periodically checks if lid is closed and puts machine to sleep. Nothing fancy
 
 Author: Phil Pirozhkov
 
+Contributors: @benbosman
+
 License: MIT

noclamshell

@@ -1,6 +1,13 @@
 #!/usr/bin/env bash
 
 CLAMSHELL=$(ioreg -r -k AppleClamshellState | grep AppleClamshellState | grep Yes)
+FLAG_FILE=$TMPDIR/noclamshell.lid.closed.and.sleeping
+
 if [ "$CLAMSHELL" ]; then
-  (pmset sleepnow &); sleep 10
+  if [[ ! -f $FLAG_FILE ]]; then
+    touch $FLAG_FILE
+    pmset sleepnow
+  fi
+else
+  rm -f $FLAG_FILE
 fi