Recursive mkdir

Lots of code for a very small fix.

For some reason (not important here), the directory was not created upon installing.
So then, I noticed that when I went to the admin page and pressed 'Save configuration'... the directories were not created. Reason being: you usually need to create a two-layer deep directory structure. And file_check_directory() does not create the directories recursively.

The solution: copying file_check_directory() into the module and changing the 'recursive' flag to TRUE :-)
(This is not the only place where that happens, by the way. filefield/fieldfile.inc has a field_file_check_directory() function for a.o. the same reason.)