TimeSync: Double-check that ntpd is busybox (#10992)

Kindle ships another implementation, with incompatible syntax...

Regression since #10935
Thanks to @yparitcher ;).

plugins/timesync.koplugin/main.lua

@@ -12,8 +12,18 @@ end
 local ntp_cmd
 -- Check if we have access to ntpd or ntpdate
 if os.execute("command -v ntpd >/dev/null") == 0 then
-    ntp_cmd = "ntpd -q -n -p pool.ntp.org"
-elseif os.execute("command -v ntpdate >/dev/null") == 0 then
+    -- Make sure it's actually busybox's implementation, as the syntax may otherwise differ...
+    -- (Of particular note, Kobo ships busybox ntpd, but not ntpdate; and Kindle ships ntpdate and !busybox ntpd).
+    local path = os.getenv("PATH") or ""
+    for p in path:gmatch("([^:]+)") do
+        local sym = lfs.symlinkattributes(p .. "/ntpd")
+        if sym and sym.mode == "link" and string.sub(sym.target, -7) == "busybox" then
+            ntp_cmd = "ntpd -q -n -p pool.ntp.org"
+            break
+        end
+    end
+end
+if not ntp_cmd and os.execute("command -v ntpdate >/dev/null") == 0 then
     ntp_cmd = "ntpdate pool.ntp.org"
 end
 if not ntp_cmd then