secure file access api (limited to user folder and world folder)

This adds minetest internal API to access files:

<pre>
<b>minetest.open_file(filename,fileloc,filetype,openmode)</b> -> handle
    filename  = plain name of file (no paths allowed)
    fileloc   = <i>user</i>     -> stored in user folder
                <i>world</i>    -> stored in current world folder
    filetype  = <i>plain</i>    -> open plain file
                <i>settings</i> -> provide settings api uppon plain file
                <i>sqlite</i>   -> provide settings api uppon sqlite file
    openmode  = <i>t</i> truncate file
                <i>w</i> append to end of file
                <i>r</i> read only
                nil -> default for filetype
</pre>

<pre>
<b>minetest.delete_file(filename,fileloc)</b> -> true/false
    filename = plain name of file (no paths allowed)
    fileloc  = <i>user</i>           -> stored in user folder
               <i>world</i>          -> stored in current world folder
</pre>

<pre>
<b>minetest.list_files(fileloc)</b>  -> list of files at specified location
(special files and files starting with "hidden_" wont be listed)

    fileloc  = <i>user</i>           -> stored in user folder
               <i>world</i>          -> stored in current world folder
</pre>

<b>FileRef</b>:Reference to a file (can be created by minetest.open_file)
methods:

<pre>
    <b>close()</b>:     save and close this file -> nil
    <b>save()</b>:      make sure file is stored to disk ->true/false
</pre>

Filetype <i>plain</i>:
<pre>
    <b>getline()</b>:     get a single line from file -> line or nil
    <b>write(data)</b>:   write some text to file -> true/false
    <b>read()</b>:        read whole file -> string or nil
    <b>seekline(num)</b>: set position in file to  line num -> true/false
</pre>

Filetype <i>settings</i> and <i>sqlite</i>:
<pre>
    <b>settings_set(key,value)</b>:         set key in settings file to value -> true/false
    <b>settings_setbool(key,boolvalue)</b>: set key to bool value return -> true/false
    <b>settings_get(key)</b>:               -> value/nil
    <b>settings_getbool(key)</b>:           -> bool value/nil
</pre>