bat: better returns when handling insufficient data

If we return "N/A" like we usually do then format string "$1$2" would
look like "N/AN/A". If "/" is returned a progressbar could be
broken. Now returns are: symbol for state "unknown", 0 for battery
charge, N/A for remaining time.

bat.lua

@@ -36,13 +36,13 @@ local function worker(format, batid)
     -- Get /proc/acpi/battery info
     local f = io.open("/proc/acpi/battery/"..batid.."/info")
     -- Handler for incompetent users
-    if not f then return {"/", "/", "/"} end
+    if not f then return {battery_state["unknown"], "0", "N/A"} end
     local infofile = f:read("*all")
     f:close()
 
     -- Check if the battery is present
     if infofile == nil or string.find(infofile, "present:[%s]+no") then
-        return {"/", "/", "/"}
+        return {battery_state["unknown"], "0", "N/A"}
     end
 
     -- Get capacity information
@@ -72,7 +72,7 @@ local function worker(format, batid)
     elseif state == "-" then
         timeleft = tonumber(remaining) / tonumber(rate)
     else
-        return { state, percent, "/" }
+        return {state, percent, "N/A"}
     end
     local hoursleft = math.floor(timeleft)
     local minutesleft = math.floor((timeleft - hoursleft) * 60 )