The current code creates directories in procfs named after interfaces,
but doesn't handle renaming.  This can result in name collisions and
consequent WARNINGs.  It also means that the interface name cannot
reliably be used to remove the directory - in fact the current code
doesn't even try, and always uses "wlan0"!

Since the name of a proc_dir_entry is embedded in it, use that when
removing it.

Add a netdev notifier to catch interface renaming, and remove and
re-add the directory at this point.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>