Put the undocumented df feature of mounting filesystems from device nodes under an ifdef. Leave enabled.
Mount filesystems without executable permissions since they should never be used.
Add a free_iovec() function to reset iovec's.
The primary purpose is to call nmount() in a loop with new iovec's so
free_iovec takes arguments by reference and resets their values.
Use nmount(2) rather than the obsolete mount(2).