Use /proc opportunistically only if it exists

This improves accuracy for systems with /proc and doesn't affect other
systems.

This leaves in place a race condition if many watchdog processes are
started in very short succession, since the pidfile is not atomically
checked and updated.  For purposes of the watchdog this is probably good
enough.  (See https://stackoverflow.com/a/688365/5419599 for more on
this.)

cfe_internal/core/watchdog/templates/watchdog.mustache

@@ -50,14 +50,25 @@ LOGFILE="/var/cfengine/watchdog.log"
 echo "$(date) Initiating watchdog $$" >> ${LOGFILE}
 
 if [ -s $PIDFILE ]; then
+    # We have a pidfile
     if ps -p $(cat $PIDFILE) > /dev/null 2>&1 ; then
         # There is a process with the PID in the file, but is it stale?
-        if [ /proc/"$(cat "$PIDFILE")" = "$(ls -1dtr "$PIDFILE" /proc/"$(cat "$PIDFILE")" | head -n 1)" ]; then
+        if [ -d /proc ]; then
+            # We can know for sure if it's stale
+            actual_process="/proc/$(cat "$PIDFILE")"
+            newer="$(ls -1dt "$PIDFILE" "$actual_process" | head -n 1)"
+            if [ "$actual_process" = "$newer" ];
+                # Pidfile is stale, ignore it
+                echo $$ > $PIDFILE
+            else
+                # Pidfile is definitely correct
+                echo "$(date) Aborting execution of watchdog $$, existing watchdog process $(cat $PIDFILE) running" >> ${LOGFILE}
+                exit 1
+            fi
+        else
+            # No /proc, pidfile shows a running process, we'll assume it's valid
             echo "$(date) Aborting execution of watchdog $$, existing watchdog process $(cat $PIDFILE) running" >> ${LOGFILE}
             exit 1
-        else
-            # pidfile is older than the process with that pid, ignore it
-            echo $$ > $PIDFILE
         fi
     else
         # No current process matching pid in file