volume: move volume functions into a lib

Also add some notification when changing the volume. Also move
sharetags to the `lib/` directory. And modify `loadrc` to be able to
load modules.

rc.lua

@@ -5,10 +5,19 @@ require("beautiful")
 require("naughty")
 
 -- Simple function to load additional LUA files from rc/.
-function loadrc(name)
+function loadrc(name, mod)
    local success
    local result
-   local path = awful.util.getdir("config") .. "/rc/" .. name .. ".lua"
+
+   -- Which file? In rc/ or in lib/?
+   local path = awful.util.getdir("config") .. "/" ..
+      (mod and "lib" or "rc") ..
+      "/" .. name .. ".lua"
+
+   -- If the module is already loaded, don't load it again
+   if mod and package.loaded[mod] then return package.loaded[mod] end
+
+   -- Execute the RC/module file
    success, result = pcall(function() return dofile(path) end)
    if not success then
       naughty.notify({ title = "Error while loading an RC file",
@@ -18,6 +27,12 @@ function loadrc(name)
 		     })
       return print("E: error loading RC file '" .. name .. "': " .. result)
    end
+
+   -- Is it a module?
+   if mod then
+      return package.loaded[mod]
+   end
+
    return result
 end