Merge branch '2118_mcext_enhancement'
* 2118_mcext_enhancement:
lib/vfs/interface.c: Added checking if super->path_element is not NULL
Use xdg-open by default in mc.ext.in if present to open files, fallback on current scheme otherwise.
src/filemanager/ext.c: Added ability to export global variables to external programs called from mc.ext file.
src/filemanager/ext.c: Code optimization. avoid few actions while %cd called from ext-file.
Ticket #2118: split exec_extention() function to few functions.