fs: include available space in percent, requested by Jon

Format string and key example: "${/home avail_p}"

README

@@ -217,7 +217,7 @@ vicious.widgets.fs
     systems, only local file systems are included by default
   - returns a table with string keys, using mount points as a base:
     {/ size_mb}, {/ size_gb}, {/ used_mb}, {/ used_gb}, {/ used_p},
-    {/ avail_mb}, {/ avail_gb}, {/home size_mb} etc.
+    {/ avail_mb}, {/ avail_gb}, {/ avail_p}, {/home size_mb} etc.
 
 vicious.widgets.dio
   - provides I/O statistics for requested storage devices

fs.lua

@@ -38,7 +38,9 @@ local function worker(format, nfs)
             helpers.uformat(fs_info, m .. " size",  s, unit)
             helpers.uformat(fs_info, m .. " used",  u, unit)
             helpers.uformat(fs_info, m .. " avail", a, unit)
-            fs_info["{" .. m .. " used_p}"] = tonumber(p)
+
+            fs_info["{" .. m .. " used_p}"]  = tonumber(p)
+            fs_info["{" .. m .. " avail_p}"] = 100 - tonumber(p)
         end
     end
     f:close()