spawn: Use awful.spawn.single_instance.

While it has limitations on process that "fork" themselves
and don't support startup notifications. It is more reliable
for everything else.

init.lua

@@ -17,7 +17,7 @@ local module,c_rules,tags_hash,settings,fallbacks = {},{class={},instance={}},{}
 local function on_selected_change(tag,data)
     if data and data.exec_once and tag.selected then
         for _,v in ipairs(type(data.exec_once) == "string" and {data.exec_once} or data.exec_once) do
-            awful.spawn.with_shell("ps -ef | grep -v grep | grep '" .. v .. "' > /dev/null || (" .. v .. ")")
+            awful.spawn.single_instance(v)
         end
     end
 end