Adding path support to load_muplugin_textdomain() function

[uglyrobot]

It always bugged me that the load_muplugin_textdomain() function didn't support a relative path, so I was forced to use the depreciated argument in load_plugin_textdomain() if I wanted my language files in a folder.

Simple backwards compatible fix to bring it in line with load_plugin_textdomain().

[nbachiyski]

(In [14186]) Make the $path argument of load_muplugin_textdomain() functional -- relative to mu-plugins. Props uglyrobot, fixes #12875