Replace buggy zed_file_create_dirs()

zed_file_create_dirs() contains a bug in checking the return value of mkdir().  This can cause the routine to exit before all of the directory components have been created.  Rather than fix the bug, this replaces zed_file_create_dirs() with mkdirp().  This cleanup was already planned, and zed_file_create_dirs() only existed because I didn't realize mkdirp() was already in tree at the time.