Update system-info macOS user agent

Add CPU temperature to `status-info`, conditionally display battery
charge percentage, and documents the commands.

system-info/system-info-macOS.sh

@@ -13,8 +13,11 @@ trap '[ -f $cache_file ] && rm $cache_file; exit' INT
 trap '[ -f $cache_file ] && rm $cache_file; exit' TERM
 
 # Check if a battery is installed.
-ioreg -w0 -l|grep BatteryInstalled &> /dev/null && \
+if ioreg -w0 -l|grep BatteryInstalled &> /dev/null; then
-  has_battery=true || has_battery=false
+  has_battery=true
+else
+  has_battery=false
+fi
 
 while true; do
   # Get the current CPU temperature.