Simplify stumpwm_pid by using only pgrep

Seems that `pgrep` exists everywhere, for example in Debian it is
installed from the same package which installs `ps` and `kill`.

util/stumpish/stumpish

@@ -40,38 +40,15 @@ fi
 
 stumpwm_pid ()
 {
-    local pid=$$
-
-    while :
-    do
-      if [ $pid -eq 1 ]
-      then
-          echo "StumpWM not found in the process tree, are you sure a graphical " 1>&2
-          echo "session is running and StumpWM is your WM? If you think this is " 1>&2
-          echo "a bug in stumpish, please report it." 1>&2
-          echo 1>&2
-          exit 1
-      elif [ -e "/proc/${pid}/comm" -a "$(cat /proc/${pid}/comm 2>/dev/null)" = "stumpwm" ]
-      then
-          STUMPWM_PID=$pid
-          break
-      elif [ -n "$(which pgrep 2>/dev/null)" ]
-      then
-          pid=$(pgrep stumpwm)
-          if [ -n $pid ]
-          then
-              STUMPWM_PID=$pid
-              break
-          else
-              pid=1
-          fi
-      elif [ -e /proc/$pid/stat ]
-      then
-          pid=$(cut -f 4 -d " " < /proc/$pid/stat)
-      else
-          pid=1
-      fi
-    done
+    STUMPWM_PID=$(pgrep stumpwm)
+
+    if [ -z "$STUMPWM_PID" ]; then
+      echo "StumpWM not found in the process tree, are you sure a graphical " 1>&2
+      echo "session is running and StumpWM is your WM? If you think this is " 1>&2
+      echo "a bug in stumpish, please report it." 1>&2
+      echo 1>&2
+      exit 1
+    fi
 }
 
 wait_result ()