Pocketbook: Launcher OS integration (#6620)

* Bring in restart koreader & shutdown device exit options
* Existing instance reused for opens via native book explorer

reader.lua

@@ -321,6 +321,14 @@ end
 
 -- Exit
 local function exitReader()
+    -- Exit code can be shoddy on some platforms due to broken library dtors calling _exit(0) from os.exit(N)
+    local ko_exit = os.getenv("KO_EXIT_CODE")
+    if ko_exit then
+        local fo = io.open(ko_exit, "w+")
+        fo:write(tostring(exit_code))
+        fo:close()
+    end
+
     local ReaderActivityIndicator =
         require("apps/reader/modules/readeractivityindicator")