checked dependencies, made some optional, document optional dependencies

profile.d/fun.sh

@@ -94,6 +94,11 @@ busy()
     delay_s=$(awk "BEGIN{
         printf \"%.3f\", $delay_ms / 1000 }")
 
+    command -v hexdump >/dev/null 2>&1 || {
+        disp E "busy: 'hexdump' is required but not installed (util-linux or bsdmainutils)."
+        return 1
+    }
+
     # Monitor /dev/urandom
     (
         hexdump -C < /dev/urandom | grep -iF --line-buffered "$pattern" | \