We use a new mutex to protect all additions/removals of drivers and
ports.  That cures a lot of insanity:
	* driver removals can't hit us in the middle of attach_driver_chain().
Old code simply dies on that.
	* port removals can't hit us in the middle of driver registration.
Again, old code dies on that.
	* driver ->detach() is allowed to block now.
	* we are guaranteed that by the time when parport_unregister_driver()
returns, all ->detach() calls are finished.  Old code did _not_ guarantee
that (read: was inherently racy since rmmod of driver could race with port
removal and get driver->detach(port) called after the module was gone).
	* we are guaranteed that driver->attach(port) won't be called
more than once.  With the old code that was a matter of luck.
	* removed piles and piles of braindead code.