luacheck: add global objects, ignore W131 only

Inspired by AwesomeWM's luacheckrc:
https://github.com/awesomeWM/awesome/blob/master/.luacheckrc

Signed-off-by: Luca Capezzuto <luca.cpz@gmail.com>

.luacheckrc

@@ -1,13 +1,27 @@
-codes = true
+-- Only allow symbols available in all Lua versions
+std = "min"
+
 allow_defined = true
 max_line_length = false
+cache = true
+
+-- Global objects defined by the C code
+read_globals = {
+    "awesome",
+    "mousegrabber",
+    "table.unpack",
+    "unpack",
+    "utf8"
+}
 
 globals = {
-    "client", "screen", "mouse"
+    "client",
+    "mouse",
+    "root",
+    "screen"
 }
 
 -- https://luacheck.readthedocs.io/en/stable/warnings.html
 ignore = {
-    "113",
     "131"
 }

helpers.lua

@@ -12,7 +12,7 @@ local io         = { lines = io.lines,
                  open  = io.open }
 local pairs  = pairs
 local rawget = rawget
-local table      = { sort  = table.sort, unpack = table.unpack }
+local tsort  = table.sort
 local unpack = unpack or table.unpack -- lua 5.1 retro-compatibility
 
 -- Lain helper functions for internal use
@@ -164,7 +164,7 @@ function helpers.spairs(t)
     local keys = {}
     for k in pairs(t) do keys[#keys+1] = k end
 
-    table.sort(keys)
+    tsort(keys)
 
     -- return the iterator function
     local i = 0